<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2025 Workiva-->
<!--r:90be6944-3777-4e60-ba1e-316699b88c2c,g:f8616bd9-d7e3-4010-af44-93a4ce2dea90-->
<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member"/>
  <link:roleRef roleURI="http://casella.com/role/COVER" xlink:type="simple" xlink:href="cwst-20250331.xsd#COVER"/>
  <link:definitionLink xlink:role="http://casella.com/role/COVER" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentInformationLineItems_d842f7a8-4adf-485a-9d74-00352bb48a98" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_DocumentInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentInformationTable_eb426570-fd38-431d-af08-43604dab049f" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_DocumentInformationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_dei_DocumentInformationLineItems_d842f7a8-4adf-485a-9d74-00352bb48a98" xlink:to="loc_dei_DocumentInformationTable_eb426570-fd38-431d-af08-43604dab049f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_ff4edda7-c2a8-4d1f-a428-c3d04bf2d886" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dei_DocumentInformationTable_eb426570-fd38-431d-af08-43604dab049f" xlink:to="loc_us-gaap_StatementClassOfStockAxis_ff4edda7-c2a8-4d1f-a428-c3d04bf2d886" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_ff4edda7-c2a8-4d1f-a428-c3d04bf2d886_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_ff4edda7-c2a8-4d1f-a428-c3d04bf2d886" xlink:to="loc_us-gaap_ClassOfStockDomain_ff4edda7-c2a8-4d1f-a428-c3d04bf2d886_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_116928e6-ed6c-416f-9b31-9447e15e45bf" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_ff4edda7-c2a8-4d1f-a428-c3d04bf2d886" xlink:to="loc_us-gaap_ClassOfStockDomain_116928e6-ed6c-416f-9b31-9447e15e45bf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_f93cb9c4-7053-4a44-b371-3eab3a301d53" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_116928e6-ed6c-416f-9b31-9447e15e45bf" xlink:to="loc_us-gaap_CommonClassAMember_f93cb9c4-7053-4a44-b371-3eab3a301d53" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_47b794f5-aa82-4bc1-9f66-030624d224f7" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_116928e6-ed6c-416f-9b31-9447e15e45bf" xlink:to="loc_us-gaap_CommonClassBMember_47b794f5-aa82-4bc1-9f66-030624d224f7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentType_a0a1fc4b-2e52-4ab0-bcb5-a068a727f025" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_DocumentType"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_d842f7a8-4adf-485a-9d74-00352bb48a98" xlink:to="loc_dei_DocumentType_a0a1fc4b-2e52-4ab0-bcb5-a068a727f025" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentQuarterlyReport_197de361-dc35-49a4-9f1d-5e87a8d9086f" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_DocumentQuarterlyReport"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_d842f7a8-4adf-485a-9d74-00352bb48a98" xlink:to="loc_dei_DocumentQuarterlyReport_197de361-dc35-49a4-9f1d-5e87a8d9086f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentPeriodEndDate_18300fea-d960-4b50-a1d1-3c2a6c9edc00" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_DocumentPeriodEndDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_d842f7a8-4adf-485a-9d74-00352bb48a98" xlink:to="loc_dei_DocumentPeriodEndDate_18300fea-d960-4b50-a1d1-3c2a6c9edc00" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentTransitionReport_31098efe-65b0-4ba4-8bb7-07815e2774e6" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_DocumentTransitionReport"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_d842f7a8-4adf-485a-9d74-00352bb48a98" xlink:to="loc_dei_DocumentTransitionReport_31098efe-65b0-4ba4-8bb7-07815e2774e6" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFileNumber_9fbd0189-7e38-4e21-ab3f-7027fbc2a8a3" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityFileNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_d842f7a8-4adf-485a-9d74-00352bb48a98" xlink:to="loc_dei_EntityFileNumber_9fbd0189-7e38-4e21-ab3f-7027fbc2a8a3" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityRegistrantName_2af6b803-2101-491e-8663-af6c5a1add6b" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityRegistrantName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_d842f7a8-4adf-485a-9d74-00352bb48a98" xlink:to="loc_dei_EntityRegistrantName_2af6b803-2101-491e-8663-af6c5a1add6b" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityIncorporationStateCountryCode_880e8534-dda7-4e87-9aff-d3e9582e999b" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityIncorporationStateCountryCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_d842f7a8-4adf-485a-9d74-00352bb48a98" xlink:to="loc_dei_EntityIncorporationStateCountryCode_880e8534-dda7-4e87-9aff-d3e9582e999b" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityTaxIdentificationNumber_b692f056-7afa-451f-a413-5968d1407871" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityTaxIdentificationNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_d842f7a8-4adf-485a-9d74-00352bb48a98" xlink:to="loc_dei_EntityTaxIdentificationNumber_b692f056-7afa-451f-a413-5968d1407871" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressAddressLine1_8be936bc-cffa-4756-bb5e-827ab76a31dd" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityAddressAddressLine1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_d842f7a8-4adf-485a-9d74-00352bb48a98" xlink:to="loc_dei_EntityAddressAddressLine1_8be936bc-cffa-4756-bb5e-827ab76a31dd" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressCityOrTown_dceb9700-e291-4da3-bf13-e2da6a1903ff" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityAddressCityOrTown"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_d842f7a8-4adf-485a-9d74-00352bb48a98" xlink:to="loc_dei_EntityAddressCityOrTown_dceb9700-e291-4da3-bf13-e2da6a1903ff" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressStateOrProvince_532a8f29-c8be-4701-9082-72ebde0e565f" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityAddressStateOrProvince"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_d842f7a8-4adf-485a-9d74-00352bb48a98" xlink:to="loc_dei_EntityAddressStateOrProvince_532a8f29-c8be-4701-9082-72ebde0e565f" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressPostalZipCode_26813a5d-1872-42a2-8260-aac5353f8d76" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityAddressPostalZipCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_d842f7a8-4adf-485a-9d74-00352bb48a98" xlink:to="loc_dei_EntityAddressPostalZipCode_26813a5d-1872-42a2-8260-aac5353f8d76" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CityAreaCode_e7cfdc94-967e-4699-9715-134a5793a49e" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_CityAreaCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_d842f7a8-4adf-485a-9d74-00352bb48a98" xlink:to="loc_dei_CityAreaCode_e7cfdc94-967e-4699-9715-134a5793a49e" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LocalPhoneNumber_39d6950b-d893-4451-9b15-078251ac8a4e" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_LocalPhoneNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_d842f7a8-4adf-485a-9d74-00352bb48a98" xlink:to="loc_dei_LocalPhoneNumber_39d6950b-d893-4451-9b15-078251ac8a4e" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_Security12bTitle_64898fd8-5ede-40c2-a7bc-27cdaa2b6e15" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_Security12bTitle"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_d842f7a8-4adf-485a-9d74-00352bb48a98" xlink:to="loc_dei_Security12bTitle_64898fd8-5ede-40c2-a7bc-27cdaa2b6e15" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_TradingSymbol_a947f3fc-a741-4f7a-b85c-e5f5f0dfbfd2" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_TradingSymbol"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_d842f7a8-4adf-485a-9d74-00352bb48a98" xlink:to="loc_dei_TradingSymbol_a947f3fc-a741-4f7a-b85c-e5f5f0dfbfd2" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_SecurityExchangeName_fd4ce4db-4079-46f9-869e-e404e0fca829" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_SecurityExchangeName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_d842f7a8-4adf-485a-9d74-00352bb48a98" xlink:to="loc_dei_SecurityExchangeName_fd4ce4db-4079-46f9-869e-e404e0fca829" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCurrentReportingStatus_52cd36d1-391e-4ed4-a2f8-b6b4657b8fc5" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityCurrentReportingStatus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_d842f7a8-4adf-485a-9d74-00352bb48a98" xlink:to="loc_dei_EntityCurrentReportingStatus_52cd36d1-391e-4ed4-a2f8-b6b4657b8fc5" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityInteractiveDataCurrent_56a8cec9-5509-475b-9f22-d867df6d4bc1" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityInteractiveDataCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_d842f7a8-4adf-485a-9d74-00352bb48a98" xlink:to="loc_dei_EntityInteractiveDataCurrent_56a8cec9-5509-475b-9f22-d867df6d4bc1" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFilerCategory_4c4b5b19-d6eb-4184-b2b6-9420d41288ac" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityFilerCategory"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_d842f7a8-4adf-485a-9d74-00352bb48a98" xlink:to="loc_dei_EntityFilerCategory_4c4b5b19-d6eb-4184-b2b6-9420d41288ac" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntitySmallBusiness_6f5640d6-e0d0-46f3-b12d-6bb163e27388" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntitySmallBusiness"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_d842f7a8-4adf-485a-9d74-00352bb48a98" xlink:to="loc_dei_EntitySmallBusiness_6f5640d6-e0d0-46f3-b12d-6bb163e27388" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityEmergingGrowthCompany_d632b4bd-345a-4099-a762-6ec02a37e043" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityEmergingGrowthCompany"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_d842f7a8-4adf-485a-9d74-00352bb48a98" xlink:to="loc_dei_EntityEmergingGrowthCompany_d632b4bd-345a-4099-a762-6ec02a37e043" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityShellCompany_c64b76d3-858e-4e27-b02e-0cb8957daa0f" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityShellCompany"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_d842f7a8-4adf-485a-9d74-00352bb48a98" xlink:to="loc_dei_EntityShellCompany_c64b76d3-858e-4e27-b02e-0cb8957daa0f" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCommonStockSharesOutstanding_484f2405-72dc-4c22-9065-7e47ad434420" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityCommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_d842f7a8-4adf-485a-9d74-00352bb48a98" xlink:to="loc_dei_EntityCommonStockSharesOutstanding_484f2405-72dc-4c22-9065-7e47ad434420" xlink:type="arc" order="23"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCentralIndexKey_27dc23c6-f7bb-4373-90ab-6768ff12a423" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityCentralIndexKey"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_d842f7a8-4adf-485a-9d74-00352bb48a98" xlink:to="loc_dei_EntityCentralIndexKey_27dc23c6-f7bb-4373-90ab-6768ff12a423" xlink:type="arc" order="24"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CurrentFiscalYearEndDate_01a633e1-b760-4507-9dda-fb2b2069597d" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_CurrentFiscalYearEndDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_d842f7a8-4adf-485a-9d74-00352bb48a98" xlink:to="loc_dei_CurrentFiscalYearEndDate_01a633e1-b760-4507-9dda-fb2b2069597d" xlink:type="arc" order="25"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentFiscalYearFocus_48f2bcb1-b8d3-4b04-8a5e-188a726b282a" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_DocumentFiscalYearFocus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_d842f7a8-4adf-485a-9d74-00352bb48a98" xlink:to="loc_dei_DocumentFiscalYearFocus_48f2bcb1-b8d3-4b04-8a5e-188a726b282a" xlink:type="arc" order="26"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentFiscalPeriodFocus_02466923-854a-45b0-af5e-08837b723e80" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_DocumentFiscalPeriodFocus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_d842f7a8-4adf-485a-9d74-00352bb48a98" xlink:to="loc_dei_DocumentFiscalPeriodFocus_02466923-854a-45b0-af5e-08837b723e80" xlink:type="arc" order="27"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_AmendmentFlag_653cbe80-c1ca-445f-a341-9c034f457daf" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_AmendmentFlag"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_d842f7a8-4adf-485a-9d74-00352bb48a98" xlink:to="loc_dei_AmendmentFlag_653cbe80-c1ca-445f-a341-9c034f457daf" xlink:type="arc" order="28"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/CONSOLIDATEDBALANCESHEETS" xlink:type="simple" xlink:href="cwst-20250331.xsd#CONSOLIDATEDBALANCESHEETS"/>
  <link:definitionLink xlink:role="http://casella.com/role/CONSOLIDATEDBALANCESHEETS" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_8c22f40c-174a-4dac-8523-fed12fa6e9be" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_3276e377-0037-4272-83a8-a925747ee958" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_8c22f40c-174a-4dac-8523-fed12fa6e9be" xlink:to="loc_us-gaap_StatementTable_3276e377-0037-4272-83a8-a925747ee958" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_f01f51e6-1bd3-469f-bf6f-e4bf86511378" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_3276e377-0037-4272-83a8-a925747ee958" xlink:to="loc_us-gaap_StatementClassOfStockAxis_f01f51e6-1bd3-469f-bf6f-e4bf86511378" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_f01f51e6-1bd3-469f-bf6f-e4bf86511378_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_f01f51e6-1bd3-469f-bf6f-e4bf86511378" xlink:to="loc_us-gaap_ClassOfStockDomain_f01f51e6-1bd3-469f-bf6f-e4bf86511378_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_c4f31e48-61ec-4131-87f1-c326e49c3b1a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_f01f51e6-1bd3-469f-bf6f-e4bf86511378" xlink:to="loc_us-gaap_ClassOfStockDomain_c4f31e48-61ec-4131-87f1-c326e49c3b1a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_0c5f6d5f-05da-45a5-8464-fad45bc4178b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_c4f31e48-61ec-4131-87f1-c326e49c3b1a" xlink:to="loc_us-gaap_CommonClassAMember_0c5f6d5f-05da-45a5-8464-fad45bc4178b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_53bf9996-2307-44f6-921b-dc452d9691bb" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_c4f31e48-61ec-4131-87f1-c326e49c3b1a" xlink:to="loc_us-gaap_CommonClassBMember_53bf9996-2307-44f6-921b-dc452d9691bb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsAbstract_218863e7-aaec-45d8-9688-f8cdce7397be" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AssetsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_8c22f40c-174a-4dac-8523-fed12fa6e9be" xlink:to="loc_us-gaap_AssetsAbstract_218863e7-aaec-45d8-9688-f8cdce7397be" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsCurrentAbstract_837fe082-c725-46fd-91ec-22b7f305ae36" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AssetsCurrentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_218863e7-aaec-45d8-9688-f8cdce7397be" xlink:to="loc_us-gaap_AssetsCurrentAbstract_837fe082-c725-46fd-91ec-22b7f305ae36" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashCashEquivalentsAndShortTermInvestments_306259a1-45c0-46c2-830b-1ca38e0d3346" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CashCashEquivalentsAndShortTermInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_837fe082-c725-46fd-91ec-22b7f305ae36" xlink:to="loc_us-gaap_CashCashEquivalentsAndShortTermInvestments_306259a1-45c0-46c2-830b-1ca38e0d3346" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableNetCurrent_2c4cc788-b42e-4f36-aeac-830d8a669005" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AccountsReceivableNetCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_837fe082-c725-46fd-91ec-22b7f305ae36" xlink:to="loc_us-gaap_AccountsReceivableNetCurrent_2c4cc788-b42e-4f36-aeac-830d8a669005" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxesReceivable_4cec7e4b-2e8a-41f7-abe1-ffedf1b4a7bd" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeTaxesReceivable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_837fe082-c725-46fd-91ec-22b7f305ae36" xlink:to="loc_us-gaap_IncomeTaxesReceivable_4cec7e4b-2e8a-41f7-abe1-ffedf1b4a7bd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrepaidExpenseCurrent_0a5306d1-bb72-463c-891a-00d9ff1f42d0" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PrepaidExpenseCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_837fe082-c725-46fd-91ec-22b7f305ae36" xlink:to="loc_us-gaap_PrepaidExpenseCurrent_0a5306d1-bb72-463c-891a-00d9ff1f42d0" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InventoryGross_a2085e2a-c80f-4157-a2ed-bf8199c334b2" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_InventoryGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_837fe082-c725-46fd-91ec-22b7f305ae36" xlink:to="loc_us-gaap_InventoryGross_a2085e2a-c80f-4157-a2ed-bf8199c334b2" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherAssetsCurrent_46e6de31-7c26-4c79-a524-457356c5fde1" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherAssetsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_837fe082-c725-46fd-91ec-22b7f305ae36" xlink:to="loc_us-gaap_OtherAssetsCurrent_46e6de31-7c26-4c79-a524-457356c5fde1" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsCurrent_0971b5d0-e52c-4e5b-9c3e-3424761df39a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AssetsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_837fe082-c725-46fd-91ec-22b7f305ae36" xlink:to="loc_us-gaap_AssetsCurrent_0971b5d0-e52c-4e5b-9c3e-3424761df39a" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization_f1f9616e-dff8-4642-b65b-526e89ef655e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_218863e7-aaec-45d8-9688-f8cdce7397be" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization_f1f9616e-dff8-4642-b65b-526e89ef655e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseRightOfUseAsset_676cbd16-00c3-4820-824c-ed6a2236ed6c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OperatingLeaseRightOfUseAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_218863e7-aaec-45d8-9688-f8cdce7397be" xlink:to="loc_us-gaap_OperatingLeaseRightOfUseAsset_676cbd16-00c3-4820-824c-ed6a2236ed6c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_8805ade2-b23f-4e51-897d-308bd8cc4b08" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_218863e7-aaec-45d8-9688-f8cdce7397be" xlink:to="loc_us-gaap_Goodwill_8805ade2-b23f-4e51-897d-308bd8cc4b08" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsNet_7e9457cf-f2bf-4732-bc27-d0f9288fdfec" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FiniteLivedIntangibleAssetsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_218863e7-aaec-45d8-9688-f8cdce7397be" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsNet_7e9457cf-f2bf-4732-bc27-d0f9288fdfec" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndInvestmentsNoncurrent_999149b9-caf7-43d1-8e2e-2d82f48bba08" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RestrictedCashAndInvestmentsNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_218863e7-aaec-45d8-9688-f8cdce7397be" xlink:to="loc_us-gaap_RestrictedCashAndInvestmentsNoncurrent_999149b9-caf7-43d1-8e2e-2d82f48bba08" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherInvestmentsAndSecuritiesAtCost_d877afae-7bf6-48da-8337-5bc768265875" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherInvestmentsAndSecuritiesAtCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_218863e7-aaec-45d8-9688-f8cdce7397be" xlink:to="loc_us-gaap_OtherInvestmentsAndSecuritiesAtCost_d877afae-7bf6-48da-8337-5bc768265875" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherAssetsNoncurrent_7f36dace-82f7-47a9-b720-8fa2ae8a1209" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherAssetsNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_218863e7-aaec-45d8-9688-f8cdce7397be" xlink:to="loc_us-gaap_OtherAssetsNoncurrent_7f36dace-82f7-47a9-b720-8fa2ae8a1209" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Assets_e9b4ff8a-f671-4cf6-924a-ef319c04846e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_Assets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_218863e7-aaec-45d8-9688-f8cdce7397be" xlink:to="loc_us-gaap_Assets_e9b4ff8a-f671-4cf6-924a-ef319c04846e" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_67006d45-8c3a-4197-81fb-b496650955e3" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LiabilitiesAndStockholdersEquityAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_8c22f40c-174a-4dac-8523-fed12fa6e9be" xlink:to="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_67006d45-8c3a-4197-81fb-b496650955e3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesCurrentAbstract_09c1d310-1d39-47e2-9bb4-169a7e6dcea5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LiabilitiesCurrentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_67006d45-8c3a-4197-81fb-b496650955e3" xlink:to="loc_us-gaap_LiabilitiesCurrentAbstract_09c1d310-1d39-47e2-9bb4-169a7e6dcea5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtAndCapitalLeaseObligationsCurrent_962c1109-fd8d-42c7-8f54-7eea9ed4e031" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LongTermDebtAndCapitalLeaseObligationsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_09c1d310-1d39-47e2-9bb4-169a7e6dcea5" xlink:to="loc_us-gaap_LongTermDebtAndCapitalLeaseObligationsCurrent_962c1109-fd8d-42c7-8f54-7eea9ed4e031" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseLiabilityCurrent_5b100d14-0d4c-4f83-b730-e844685df514" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OperatingLeaseLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_09c1d310-1d39-47e2-9bb4-169a7e6dcea5" xlink:to="loc_us-gaap_OperatingLeaseLiabilityCurrent_5b100d14-0d4c-4f83-b730-e844685df514" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsPayableCurrent_f30b2fb5-ccaf-4d9d-ad29-c3fe37f8acb7" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AccountsPayableCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_09c1d310-1d39-47e2-9bb4-169a7e6dcea5" xlink:to="loc_us-gaap_AccountsPayableCurrent_f30b2fb5-ccaf-4d9d-ad29-c3fe37f8acb7" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeRelatedLiabilitiesCurrent_168e95e9-971d-43ea-a656-b21e8a8d1a9d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EmployeeRelatedLiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_09c1d310-1d39-47e2-9bb4-169a7e6dcea5" xlink:to="loc_us-gaap_EmployeeRelatedLiabilitiesCurrent_168e95e9-971d-43ea-a656-b21e8a8d1a9d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestPayableCurrent_af01cb40-eff0-4a75-a13a-2808edb6ce76" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_InterestPayableCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_09c1d310-1d39-47e2-9bb4-169a7e6dcea5" xlink:to="loc_us-gaap_InterestPayableCurrent_af01cb40-eff0-4a75-a13a-2808edb6ce76" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityCurrent_ac11f692-fd61-4554-abe6-393dc69ad69e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ContractWithCustomerLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_09c1d310-1d39-47e2-9bb4-169a7e6dcea5" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityCurrent_ac11f692-fd61-4554-abe6-393dc69ad69e" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccruedCappingClosurePostClosureAndEnvironmentalCosts_63ede580-8fd5-4276-8cbd-cb841472725f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AccruedCappingClosurePostClosureAndEnvironmentalCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_09c1d310-1d39-47e2-9bb4-169a7e6dcea5" xlink:to="loc_us-gaap_AccruedCappingClosurePostClosureAndEnvironmentalCosts_63ede580-8fd5-4276-8cbd-cb841472725f" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherAccruedLiabilitiesCurrent_b8d41424-a134-4ca2-bf48-f90bc118d12a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherAccruedLiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_09c1d310-1d39-47e2-9bb4-169a7e6dcea5" xlink:to="loc_us-gaap_OtherAccruedLiabilitiesCurrent_b8d41424-a134-4ca2-bf48-f90bc118d12a" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesCurrent_01dece0d-7eec-4cad-b590-c7af3f11573b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_09c1d310-1d39-47e2-9bb4-169a7e6dcea5" xlink:to="loc_us-gaap_LiabilitiesCurrent_01dece0d-7eec-4cad-b590-c7af3f11573b" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtAndCapitalLeaseObligations_7f6dcfc8-2ed8-494d-a2fd-ad59d19369f4" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LongTermDebtAndCapitalLeaseObligations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_67006d45-8c3a-4197-81fb-b496650955e3" xlink:to="loc_us-gaap_LongTermDebtAndCapitalLeaseObligations_7f6dcfc8-2ed8-494d-a2fd-ad59d19369f4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseLiabilityNoncurrent_6f636326-3d51-4187-8513-b04a73890831" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OperatingLeaseLiabilityNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_67006d45-8c3a-4197-81fb-b496650955e3" xlink:to="loc_us-gaap_OperatingLeaseLiabilityNoncurrent_6f636326-3d51-4187-8513-b04a73890831" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccruedCappingClosurePostClosureAndEnvironmentalCostsNoncurrent_023a785b-8cb1-48fb-91df-38cbf511f54e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AccruedCappingClosurePostClosureAndEnvironmentalCostsNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_67006d45-8c3a-4197-81fb-b496650955e3" xlink:to="loc_us-gaap_AccruedCappingClosurePostClosureAndEnvironmentalCostsNoncurrent_023a785b-8cb1-48fb-91df-38cbf511f54e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredIncomeTaxLiabilitiesNet_a9c03364-bf8d-4419-9993-6000381c9903" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DeferredIncomeTaxLiabilitiesNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_67006d45-8c3a-4197-81fb-b496650955e3" xlink:to="loc_us-gaap_DeferredIncomeTaxLiabilitiesNet_a9c03364-bf8d-4419-9993-6000381c9903" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherLiabilitiesNoncurrent_3fa4c985-7026-4965-9618-9698d989eb75" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherLiabilitiesNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_67006d45-8c3a-4197-81fb-b496650955e3" xlink:to="loc_us-gaap_OtherLiabilitiesNoncurrent_3fa4c985-7026-4965-9618-9698d989eb75" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommitmentsAndContingencies_83641126-dc81-4c70-880e-d96c7c6a13c1" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommitmentsAndContingencies"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_67006d45-8c3a-4197-81fb-b496650955e3" xlink:to="loc_us-gaap_CommitmentsAndContingencies_83641126-dc81-4c70-880e-d96c7c6a13c1" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityAbstract_5f93fc80-dbc7-425a-8d09-f3f7725f8bca" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StockholdersEquityAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_67006d45-8c3a-4197-81fb-b496650955e3" xlink:to="loc_us-gaap_StockholdersEquityAbstract_5f93fc80-dbc7-425a-8d09-f3f7725f8bca" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockValue_fc23b15b-8bd3-45c8-904b-a90d727c85de" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonStockValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_5f93fc80-dbc7-425a-8d09-f3f7725f8bca" xlink:to="loc_us-gaap_CommonStockValue_fc23b15b-8bd3-45c8-904b-a90d727c85de" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalCommonStock_826adbe6-9928-49b9-bde7-0a01dca0095f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AdditionalPaidInCapitalCommonStock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_5f93fc80-dbc7-425a-8d09-f3f7725f8bca" xlink:to="loc_us-gaap_AdditionalPaidInCapitalCommonStock_826adbe6-9928-49b9-bde7-0a01dca0095f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsAccumulatedDeficit_0e244c86-3542-4312-80eb-00d63108cd12" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RetainedEarningsAccumulatedDeficit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_5f93fc80-dbc7-425a-8d09-f3f7725f8bca" xlink:to="loc_us-gaap_RetainedEarningsAccumulatedDeficit_0e244c86-3542-4312-80eb-00d63108cd12" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_a66c3148-1a8a-40df-bb97-ea3e20f9ef04" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_5f93fc80-dbc7-425a-8d09-f3f7725f8bca" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_a66c3148-1a8a-40df-bb97-ea3e20f9ef04" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_484585a5-5b64-4260-a6f9-1f73a155a7fe" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_5f93fc80-dbc7-425a-8d09-f3f7725f8bca" xlink:to="loc_us-gaap_StockholdersEquity_484585a5-5b64-4260-a6f9-1f73a155a7fe" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAndStockholdersEquity_fa9063d9-9963-45bd-af59-5e54393d788e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LiabilitiesAndStockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_67006d45-8c3a-4197-81fb-b496650955e3" xlink:to="loc_us-gaap_LiabilitiesAndStockholdersEquity_fa9063d9-9963-45bd-af59-5e54393d788e" xlink:type="arc" order="9"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/CONSOLIDATEDBALANCESHEETSParenthetical" xlink:type="simple" xlink:href="cwst-20250331.xsd#CONSOLIDATEDBALANCESHEETSParenthetical"/>
  <link:definitionLink xlink:role="http://casella.com/role/CONSOLIDATEDBALANCESHEETSParenthetical" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_eb6cafd0-a992-4938-ae15-22ce041da522" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_ad5f8c8c-850a-4e52-a9fc-32b6450a7089" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_eb6cafd0-a992-4938-ae15-22ce041da522" xlink:to="loc_us-gaap_StatementTable_ad5f8c8c-850a-4e52-a9fc-32b6450a7089" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_ee3a7262-ffbe-4e71-bfad-28eb0b0b838c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_ad5f8c8c-850a-4e52-a9fc-32b6450a7089" xlink:to="loc_us-gaap_StatementClassOfStockAxis_ee3a7262-ffbe-4e71-bfad-28eb0b0b838c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_ee3a7262-ffbe-4e71-bfad-28eb0b0b838c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_ee3a7262-ffbe-4e71-bfad-28eb0b0b838c" xlink:to="loc_us-gaap_ClassOfStockDomain_ee3a7262-ffbe-4e71-bfad-28eb0b0b838c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_ae3d8b85-412d-498a-84b0-a19e9cdd87a5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_ee3a7262-ffbe-4e71-bfad-28eb0b0b838c" xlink:to="loc_us-gaap_ClassOfStockDomain_ae3d8b85-412d-498a-84b0-a19e9cdd87a5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_2d665f9d-316c-46a8-9445-001801c2ccc4" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_ae3d8b85-412d-498a-84b0-a19e9cdd87a5" xlink:to="loc_us-gaap_CommonClassAMember_2d665f9d-316c-46a8-9445-001801c2ccc4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_306c7e7e-367a-4bd0-8bc7-7ab17afe3a7d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_ae3d8b85-412d-498a-84b0-a19e9cdd87a5" xlink:to="loc_us-gaap_CommonClassBMember_306c7e7e-367a-4bd0-8bc7-7ab17afe3a7d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent_34358c3e-b092-4222-8d25-416a40a6eb97" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_eb6cafd0-a992-4938-ae15-22ce041da522" xlink:to="loc_us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent_34358c3e-b092-4222-8d25-416a40a6eb97" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAccumulatedDepreciationAndAmortization_292d5456-3765-4fc1-82bf-44e29062a9a9" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAccumulatedDepreciationAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_eb6cafd0-a992-4938-ae15-22ce041da522" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAccumulatedDepreciationAndAmortization_292d5456-3765-4fc1-82bf-44e29062a9a9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_61a4ffe6-7949-44de-b5ad-d676dc022c28" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_eb6cafd0-a992-4938-ae15-22ce041da522" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_61a4ffe6-7949-44de-b5ad-d676dc022c28" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesAuthorized_3fb95cfc-83bf-4485-852a-7a6491b0cda6" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_eb6cafd0-a992-4938-ae15-22ce041da522" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_3fb95cfc-83bf-4485-852a-7a6491b0cda6" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesIssued_7d355715-ad44-4af5-99a4-bc2904fc1818" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_eb6cafd0-a992-4938-ae15-22ce041da522" xlink:to="loc_us-gaap_CommonStockSharesIssued_7d355715-ad44-4af5-99a4-bc2904fc1818" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_2ec27305-9c13-40d4-b512-25fc9d18e038" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_eb6cafd0-a992-4938-ae15-22ce041da522" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_2ec27305-9c13-40d4-b512-25fc9d18e038" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_StockholdersEquityNumberOfVotesEachShare_f46a903f-2845-4dad-a451-ddd5d94cb48c" xlink:href="cwst-20250331.xsd#cwst_StockholdersEquityNumberOfVotesEachShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_eb6cafd0-a992-4938-ae15-22ce041da522" xlink:to="loc_cwst_StockholdersEquityNumberOfVotesEachShare_f46a903f-2845-4dad-a451-ddd5d94cb48c" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/UNAUDITEDCONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITY" xlink:type="simple" xlink:href="cwst-20250331.xsd#UNAUDITEDCONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITY"/>
  <link:definitionLink xlink:role="http://casella.com/role/UNAUDITEDCONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITY" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_a01c8f44-30e1-4bfd-949b-5eb79c2ef3d3" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_97f451a3-9fe9-4622-af83-032f135bedca" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_a01c8f44-30e1-4bfd-949b-5eb79c2ef3d3" xlink:to="loc_us-gaap_StatementTable_97f451a3-9fe9-4622-af83-032f135bedca" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_3e1a0db0-9b9d-4d4d-8c4e-a8b69e61e632" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_97f451a3-9fe9-4622-af83-032f135bedca" xlink:to="loc_us-gaap_StatementClassOfStockAxis_3e1a0db0-9b9d-4d4d-8c4e-a8b69e61e632" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_3e1a0db0-9b9d-4d4d-8c4e-a8b69e61e632_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_3e1a0db0-9b9d-4d4d-8c4e-a8b69e61e632" xlink:to="loc_us-gaap_ClassOfStockDomain_3e1a0db0-9b9d-4d4d-8c4e-a8b69e61e632_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_0e59c344-189b-4054-a3cb-41b7f37eca6a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_3e1a0db0-9b9d-4d4d-8c4e-a8b69e61e632" xlink:to="loc_us-gaap_ClassOfStockDomain_0e59c344-189b-4054-a3cb-41b7f37eca6a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_886d6663-8477-434a-9b42-d39554d456b6" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_0e59c344-189b-4054-a3cb-41b7f37eca6a" xlink:to="loc_us-gaap_CommonClassAMember_886d6663-8477-434a-9b42-d39554d456b6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_929e2553-851a-458d-b974-59b751074a33" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_0e59c344-189b-4054-a3cb-41b7f37eca6a" xlink:to="loc_us-gaap_CommonClassBMember_929e2553-851a-458d-b974-59b751074a33" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_46cdd3ed-67f6-4132-bbcf-823e5658306e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_97f451a3-9fe9-4622-af83-032f135bedca" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_46cdd3ed-67f6-4132-bbcf-823e5658306e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_46cdd3ed-67f6-4132-bbcf-823e5658306e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_46cdd3ed-67f6-4132-bbcf-823e5658306e" xlink:to="loc_us-gaap_EquityComponentDomain_46cdd3ed-67f6-4132-bbcf-823e5658306e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_65946a9f-4734-47e8-9667-ca76c519859d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_46cdd3ed-67f6-4132-bbcf-823e5658306e" xlink:to="loc_us-gaap_EquityComponentDomain_65946a9f-4734-47e8-9667-ca76c519859d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_5aa985ca-6d66-48a8-8a5a-a8ded0a1d2de" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_65946a9f-4734-47e8-9667-ca76c519859d" xlink:to="loc_us-gaap_CommonStockMember_5aa985ca-6d66-48a8-8a5a-a8ded0a1d2de" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_c7d4e9a2-31f6-4ab4-a260-9ca558cf1f4a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_65946a9f-4734-47e8-9667-ca76c519859d" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_c7d4e9a2-31f6-4ab4-a260-9ca558cf1f4a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_e209e189-0530-4b2a-95c2-b0f515ded99f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_65946a9f-4734-47e8-9667-ca76c519859d" xlink:to="loc_us-gaap_RetainedEarningsMember_e209e189-0530-4b2a-95c2-b0f515ded99f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_6c4494e9-9743-4c07-8ff6-a2826256eab8" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_65946a9f-4734-47e8-9667-ca76c519859d" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_6c4494e9-9743-4c07-8ff6-a2826256eab8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_de7914c5-4eaa-408a-ac0e-555f6fe7264c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_a01c8f44-30e1-4bfd-949b-5eb79c2ef3d3" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_de7914c5-4eaa-408a-ac0e-555f6fe7264c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_bac5254b-859f-40aa-b2cd-07cc3be885dc" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_de7914c5-4eaa-408a-ac0e-555f6fe7264c" xlink:to="loc_us-gaap_StockholdersEquity_bac5254b-859f-40aa-b2cd-07cc3be885dc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_a7ad90e0-dfd7-4302-acbe-abbdfc73ba9d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_de7914c5-4eaa-408a-ac0e-555f6fe7264c" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_a7ad90e0-dfd7-4302-acbe-abbdfc73ba9d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation_dbf7af70-2f33-4e11-ae61-260fac455a21" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_de7914c5-4eaa-408a-ac0e-555f6fe7264c" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation_dbf7af70-2f33-4e11-ae61-260fac455a21" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation_1273efc4-1483-4114-bb30-92de6518f7e0" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_de7914c5-4eaa-408a-ac0e-555f6fe7264c" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation_1273efc4-1483-4114-bb30-92de6518f7e0" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_ebf24370-abe8-45d9-a256-9ce0fc21b7f2" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_de7914c5-4eaa-408a-ac0e-555f6fe7264c" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_ebf24370-abe8-45d9-a256-9ce0fc21b7f2" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_9deedbee-cfcb-42ea-b03a-afb5c4d60cd1" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_de7914c5-4eaa-408a-ac0e-555f6fe7264c" xlink:to="loc_us-gaap_NetIncomeLoss_9deedbee-cfcb-42ea-b03a-afb5c4d60cd1" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_4db1f15e-70c8-4d36-8e95-0018b32d1be9" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_de7914c5-4eaa-408a-ac0e-555f6fe7264c" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_4db1f15e-70c8-4d36-8e95-0018b32d1be9" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_04c39e33-81b9-41a8-aa85-8f3de46963ef" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StockholdersEquity"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_1c843a21-b5e9-4aa4-8de8-eafb30d831a5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonStockSharesOutstanding"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/BASISOFPRESENTATIONDetails" xlink:type="simple" xlink:href="cwst-20250331.xsd#BASISOFPRESENTATIONDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/BASISOFPRESENTATIONDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_14ef5522-8ba9-4494-85a1-10bf4f181028" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_02a064e1-e78c-4dcc-99aa-4898f03b5eb2" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_14ef5522-8ba9-4494-85a1-10bf4f181028" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_02a064e1-e78c-4dcc-99aa-4898f03b5eb2" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_eda6ebf6-fb93-4ef7-a586-0a743ee1a724" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_02a064e1-e78c-4dcc-99aa-4898f03b5eb2" xlink:to="loc_srt_ProductOrServiceAxis_eda6ebf6-fb93-4ef7-a586-0a743ee1a724" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_eda6ebf6-fb93-4ef7-a586-0a743ee1a724_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_eda6ebf6-fb93-4ef7-a586-0a743ee1a724" xlink:to="loc_srt_ProductsAndServicesDomain_eda6ebf6-fb93-4ef7-a586-0a743ee1a724_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_e8037b7f-ac6f-4911-9b76-5fc561636272" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_eda6ebf6-fb93-4ef7-a586-0a743ee1a724" xlink:to="loc_srt_ProductsAndServicesDomain_e8037b7f-ac6f-4911-9b76-5fc561636272" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_SolidWasteOperationsMember_60bc989c-ef99-48a6-8d13-b3b57cafdf5d" xlink:href="cwst-20250331.xsd#cwst_SolidWasteOperationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_e8037b7f-ac6f-4911-9b76-5fc561636272" xlink:to="loc_cwst_SolidWasteOperationsMember_60bc989c-ef99-48a6-8d13-b3b57cafdf5d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfStatesInWhichEntityOperates_39896744-0bca-42b3-948a-624e9b025d49" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_NumberOfStatesInWhichEntityOperates"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_14ef5522-8ba9-4494-85a1-10bf4f181028" xlink:to="loc_us-gaap_NumberOfStatesInWhichEntityOperates_39896744-0bca-42b3-948a-624e9b025d49" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfOperatingSegments_1e9dcbc4-3491-4aca-8baa-b5e72bf2e10f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_NumberOfOperatingSegments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_14ef5522-8ba9-4494-85a1-10bf4f181028" xlink:to="loc_us-gaap_NumberOfOperatingSegments_1e9dcbc4-3491-4aca-8baa-b5e72bf2e10f" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/REVENUERECOGNITIONDisaggregationofRevenueDetails" xlink:type="simple" xlink:href="cwst-20250331.xsd#REVENUERECOGNITIONDisaggregationofRevenueDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/REVENUERECOGNITIONDisaggregationofRevenueDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_84a4cee8-8a71-4eaa-977e-71fab6fff9bf" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_986afea8-e349-442a-9439-0c04d18ddd3a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_84a4cee8-8a71-4eaa-977e-71fab6fff9bf" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_986afea8-e349-442a-9439-0c04d18ddd3a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_0a0c213e-6818-4af5-8255-5b9ea0a96855" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_986afea8-e349-442a-9439-0c04d18ddd3a" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_0a0c213e-6818-4af5-8255-5b9ea0a96855" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_0a0c213e-6818-4af5-8255-5b9ea0a96855_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_0a0c213e-6818-4af5-8255-5b9ea0a96855" xlink:to="loc_us-gaap_SegmentDomain_0a0c213e-6818-4af5-8255-5b9ea0a96855_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_c429f185-99ec-4ff1-a090-0574ea2ab7e6" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_0a0c213e-6818-4af5-8255-5b9ea0a96855" xlink:to="loc_us-gaap_SegmentDomain_c429f185-99ec-4ff1-a090-0574ea2ab7e6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_EasternRegionMember_a5226106-8ae5-452f-9887-a23bc9514cb3" xlink:href="cwst-20250331.xsd#cwst_EasternRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_c429f185-99ec-4ff1-a090-0574ea2ab7e6" xlink:to="loc_cwst_EasternRegionMember_a5226106-8ae5-452f-9887-a23bc9514cb3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_WesternRegionMember_3d3003cf-bcf5-4f6e-84f4-d02054295a0a" xlink:href="cwst-20250331.xsd#cwst_WesternRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_c429f185-99ec-4ff1-a090-0574ea2ab7e6" xlink:to="loc_cwst_WesternRegionMember_3d3003cf-bcf5-4f6e-84f4-d02054295a0a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_MidAtlanticRegionMember_4e0a335b-7792-4517-abb1-69ec18468efd" xlink:href="cwst-20250331.xsd#cwst_MidAtlanticRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_c429f185-99ec-4ff1-a090-0574ea2ab7e6" xlink:to="loc_cwst_MidAtlanticRegionMember_4e0a335b-7792-4517-abb1-69ec18468efd" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_ResourceSolutionsRegionMember_99be1f62-6539-44d3-a358-a859225c0133" xlink:href="cwst-20250331.xsd#cwst_ResourceSolutionsRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_c429f185-99ec-4ff1-a090-0574ea2ab7e6" xlink:to="loc_cwst_ResourceSolutionsRegionMember_99be1f62-6539-44d3-a358-a859225c0133" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_bb28553d-71d2-46e8-8419-3f2060fbeee6" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_986afea8-e349-442a-9439-0c04d18ddd3a" xlink:to="loc_srt_ProductOrServiceAxis_bb28553d-71d2-46e8-8419-3f2060fbeee6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_bb28553d-71d2-46e8-8419-3f2060fbeee6_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_bb28553d-71d2-46e8-8419-3f2060fbeee6" xlink:to="loc_srt_ProductsAndServicesDomain_bb28553d-71d2-46e8-8419-3f2060fbeee6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_ada92c5e-a174-4bf1-950e-c963bd0f64b1" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_bb28553d-71d2-46e8-8419-3f2060fbeee6" xlink:to="loc_srt_ProductsAndServicesDomain_ada92c5e-a174-4bf1-950e-c963bd0f64b1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_CollectionMember_d05dcadc-4a54-4610-8a18-f456fb0ebb93" xlink:href="cwst-20250331.xsd#cwst_CollectionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_ada92c5e-a174-4bf1-950e-c963bd0f64b1" xlink:to="loc_cwst_CollectionMember_d05dcadc-4a54-4610-8a18-f456fb0ebb93" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_LandfillRevenueMember_11144b21-939a-447a-b605-18bf2c89ad62" xlink:href="cwst-20250331.xsd#cwst_LandfillRevenueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_ada92c5e-a174-4bf1-950e-c963bd0f64b1" xlink:to="loc_cwst_LandfillRevenueMember_11144b21-939a-447a-b605-18bf2c89ad62" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_TransferStationMember_e923a16f-7413-41c2-acda-a3e1be562123" xlink:href="cwst-20250331.xsd#cwst_TransferStationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_ada92c5e-a174-4bf1-950e-c963bd0f64b1" xlink:to="loc_cwst_TransferStationMember_e923a16f-7413-41c2-acda-a3e1be562123" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_TransportationMember_d0128fb8-1ed2-4f53-9e32-f91f156fb2fb" xlink:href="cwst-20250331.xsd#cwst_TransportationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_ada92c5e-a174-4bf1-950e-c963bd0f64b1" xlink:to="loc_cwst_TransportationMember_d0128fb8-1ed2-4f53-9e32-f91f156fb2fb" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_LandfillGasToEnergyMember_2b3acae6-e4aa-48f7-985e-74766bd22776" xlink:href="cwst-20250331.xsd#cwst_LandfillGasToEnergyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_ada92c5e-a174-4bf1-950e-c963bd0f64b1" xlink:to="loc_cwst_LandfillGasToEnergyMember_2b3acae6-e4aa-48f7-985e-74766bd22776" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_ProcessingServicesMember_58b18e09-4260-4ab1-a6ee-598ba6d5e867" xlink:href="cwst-20250331.xsd#cwst_ProcessingServicesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_ada92c5e-a174-4bf1-950e-c963bd0f64b1" xlink:to="loc_cwst_ProcessingServicesMember_58b18e09-4260-4ab1-a6ee-598ba6d5e867" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_NationalAccountsMember_9dec4f49-f1d7-4e46-a57b-2582d92f7fdd" xlink:href="cwst-20250331.xsd#cwst_NationalAccountsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_ada92c5e-a174-4bf1-950e-c963bd0f64b1" xlink:to="loc_cwst_NationalAccountsMember_9dec4f49-f1d7-4e46-a57b-2582d92f7fdd" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_9cab4cd0-02e0-4844-b824-bbaea4dedc2f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_TimingOfTransferOfGoodOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_986afea8-e349-442a-9439-0c04d18ddd3a" xlink:to="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_9cab4cd0-02e0-4844-b824-bbaea4dedc2f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_9cab4cd0-02e0-4844-b824-bbaea4dedc2f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_TimingOfTransferOfGoodOrServiceDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_9cab4cd0-02e0-4844-b824-bbaea4dedc2f" xlink:to="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_9cab4cd0-02e0-4844-b824-bbaea4dedc2f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_cc2f33bd-e893-4ecc-9dcf-32b67c1e7a03" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_TimingOfTransferOfGoodOrServiceDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_9cab4cd0-02e0-4844-b824-bbaea4dedc2f" xlink:to="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_cc2f33bd-e893-4ecc-9dcf-32b67c1e7a03" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransferredAtPointInTimeMember_7df03528-d450-4a54-ba2e-4dc898a8fc1b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_TransferredAtPointInTimeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_cc2f33bd-e893-4ecc-9dcf-32b67c1e7a03" xlink:to="loc_us-gaap_TransferredAtPointInTimeMember_7df03528-d450-4a54-ba2e-4dc898a8fc1b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransferredOverTimeMember_c6018efe-7e50-43eb-bfc5-466feca0c6ec" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_TransferredOverTimeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_cc2f33bd-e893-4ecc-9dcf-32b67c1e7a03" xlink:to="loc_us-gaap_TransferredOverTimeMember_c6018efe-7e50-43eb-bfc5-466feca0c6ec" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_d76e587d-0615-4c61-9a9e-404cd57f38c8" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_84a4cee8-8a71-4eaa-977e-71fab6fff9bf" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_d76e587d-0615-4c61-9a9e-404cd57f38c8" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/REVENUERECOGNITIONNarrativeDetails" xlink:type="simple" xlink:href="cwst-20250331.xsd#REVENUERECOGNITIONNarrativeDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/REVENUERECOGNITIONNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_8bbc870c-b35d-42a6-a255-e1460dd4c2e7" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_b0345713-6d14-4311-b2f7-18d3c49dc501" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_8bbc870c-b35d-42a6-a255-e1460dd4c2e7" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_b0345713-6d14-4311-b2f7-18d3c49dc501" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_b82fe2d3-92d7-4946-9734-8388f77faafc" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_b0345713-6d14-4311-b2f7-18d3c49dc501" xlink:to="loc_srt_ProductOrServiceAxis_b82fe2d3-92d7-4946-9734-8388f77faafc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_b82fe2d3-92d7-4946-9734-8388f77faafc_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_b82fe2d3-92d7-4946-9734-8388f77faafc" xlink:to="loc_srt_ProductsAndServicesDomain_b82fe2d3-92d7-4946-9734-8388f77faafc_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_cc29cde9-d1a9-403a-a0e4-3027e9a22117" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_b82fe2d3-92d7-4946-9734-8388f77faafc" xlink:to="loc_srt_ProductsAndServicesDomain_cc29cde9-d1a9-403a-a0e4-3027e9a22117" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_RebateRecycledOrReturnedOrganicMaterialsMember_b058af59-5d02-4a74-a753-d40b054b1e17" xlink:href="cwst-20250331.xsd#cwst_RebateRecycledOrReturnedOrganicMaterialsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_cc29cde9-d1a9-403a-a0e4-3027e9a22117" xlink:to="loc_cwst_RebateRecycledOrReturnedOrganicMaterialsMember_b058af59-5d02-4a74-a753-d40b054b1e17" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_e9258f01-5486-4e4c-bf6b-4ef0f3d28836" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_8bbc870c-b35d-42a6-a255-e1460dd4c2e7" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_e9258f01-5486-4e4c-bf6b-4ef0f3d28836" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerPerformanceObligationSatisfiedInPreviousPeriod_c700bf61-99ab-46f6-acb4-d3511214fc50" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ContractWithCustomerPerformanceObligationSatisfiedInPreviousPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_8bbc870c-b35d-42a6-a255-e1460dd4c2e7" xlink:to="loc_us-gaap_ContractWithCustomerPerformanceObligationSatisfiedInPreviousPeriod_c700bf61-99ab-46f6-acb4-d3511214fc50" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerAssetGrossCurrent_49d2c4d7-75b4-4c8f-843d-ba7580a1f644" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ContractWithCustomerAssetGrossCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_8bbc870c-b35d-42a6-a255-e1460dd4c2e7" xlink:to="loc_us-gaap_ContractWithCustomerAssetGrossCurrent_49d2c4d7-75b4-4c8f-843d-ba7580a1f644" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityCurrent_2a84d529-fee4-4695-ab14-dd816965612d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ContractWithCustomerLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_8bbc870c-b35d-42a6-a255-e1460dd4c2e7" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityCurrent_2a84d529-fee4-4695-ab14-dd816965612d" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/BUSINESSCOMBINATIONSNarrativeDetails" xlink:type="simple" xlink:href="cwst-20250331.xsd#BUSINESSCOMBINATIONSNarrativeDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/BUSINESSCOMBINATIONSNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_5aa0294b-4db4-4ccd-be25-05444eee5373" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_f15fdbe8-f85b-48ac-a4bf-7aa3036116de" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_5aa0294b-4db4-4ccd-be25-05444eee5373" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_f15fdbe8-f85b-48ac-a4bf-7aa3036116de" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_cec13bbd-13b4-4e3a-9b6b-d756b3011fd0" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_f15fdbe8-f85b-48ac-a4bf-7aa3036116de" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_cec13bbd-13b4-4e3a-9b6b-d756b3011fd0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_cec13bbd-13b4-4e3a-9b6b-d756b3011fd0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_cec13bbd-13b4-4e3a-9b6b-d756b3011fd0" xlink:to="loc_us-gaap_SegmentDomain_cec13bbd-13b4-4e3a-9b6b-d756b3011fd0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_d6b3c096-4be4-4796-b4d6-56ca52dfb4e5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_cec13bbd-13b4-4e3a-9b6b-d756b3011fd0" xlink:to="loc_us-gaap_SegmentDomain_d6b3c096-4be4-4796-b4d6-56ca52dfb4e5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_MidAtlanticRegionMember_bce97dfe-1179-443d-8bf6-b649bd865229" xlink:href="cwst-20250331.xsd#cwst_MidAtlanticRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_d6b3c096-4be4-4796-b4d6-56ca52dfb4e5" xlink:to="loc_cwst_MidAtlanticRegionMember_bce97dfe-1179-443d-8bf6-b649bd865229" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_649c0f76-4a10-4600-aa86-861f18840351" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_f15fdbe8-f85b-48ac-a4bf-7aa3036116de" xlink:to="loc_srt_RangeAxis_649c0f76-4a10-4600-aa86-861f18840351" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_649c0f76-4a10-4600-aa86-861f18840351_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_649c0f76-4a10-4600-aa86-861f18840351" xlink:to="loc_srt_RangeMember_649c0f76-4a10-4600-aa86-861f18840351_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_d159108b-c4c7-46f1-b954-154cc138feab" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_649c0f76-4a10-4600-aa86-861f18840351" xlink:to="loc_srt_RangeMember_d159108b-c4c7-46f1-b954-154cc138feab" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_ea8af300-0f4e-4e9b-878f-ddf2196b9ca2" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_d159108b-c4c7-46f1-b954-154cc138feab" xlink:to="loc_srt_MinimumMember_ea8af300-0f4e-4e9b-878f-ddf2196b9ca2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_50ca6d10-11be-4cb6-9a97-9f67f77dd740" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_d159108b-c4c7-46f1-b954-154cc138feab" xlink:to="loc_srt_MaximumMember_50ca6d10-11be-4cb6-9a97-9f67f77dd740" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfBusinessesAcquired_65ba64bc-213b-4ea0-9482-ce83b298eeef" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_NumberOfBusinessesAcquired"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_5aa0294b-4db4-4ccd-be25-05444eee5373" xlink:to="loc_us-gaap_NumberOfBusinessesAcquired_65ba64bc-213b-4ea0-9482-ce83b298eeef" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_0c583057-2ed8-4a4b-b2c3-d9e3a80907c1" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FiniteLivedIntangibleAssetUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_5aa0294b-4db4-4ccd-be25-05444eee5373" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_0c583057-2ed8-4a4b-b2c3-d9e3a80907c1" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/BUSINESSCOMBINATIONSScheduleofPurchasePricePaidforAcquisitionsDetail" xlink:type="simple" xlink:href="cwst-20250331.xsd#BUSINESSCOMBINATIONSScheduleofPurchasePricePaidforAcquisitionsDetail"/>
  <link:definitionLink xlink:role="http://casella.com/role/BUSINESSCOMBINATIONSScheduleofPurchasePricePaidforAcquisitionsDetail" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_94f83b50-0f5f-40b4-a1ed-cc8dfb197922" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_4e9e9f97-7265-41af-8de0-9e3fdc18d700" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_94f83b50-0f5f-40b4-a1ed-cc8dfb197922" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_4e9e9f97-7265-41af-8de0-9e3fdc18d700" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_49dbb556-86b3-46f3-a5b3-50b8508a1d6f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_4e9e9f97-7265-41af-8de0-9e3fdc18d700" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_49dbb556-86b3-46f3-a5b3-50b8508a1d6f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_49dbb556-86b3-46f3-a5b3-50b8508a1d6f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_49dbb556-86b3-46f3-a5b3-50b8508a1d6f" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_49dbb556-86b3-46f3-a5b3-50b8508a1d6f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_c2bcb014-24d8-40da-b226-bf1395fb2430" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_49dbb556-86b3-46f3-a5b3-50b8508a1d6f" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_c2bcb014-24d8-40da-b226-bf1395fb2430" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_TransferStationsAcquisitionMember_f4b5e8cd-7803-420a-b179-c37653f39b73" xlink:href="cwst-20250331.xsd#cwst_TransferStationsAcquisitionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_c2bcb014-24d8-40da-b226-bf1395fb2430" xlink:to="loc_cwst_TransferStationsAcquisitionMember_f4b5e8cd-7803-420a-b179-c37653f39b73" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_2dacb297-8a28-46c2-b995-2fa28047a400" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_4e9e9f97-7265-41af-8de0-9e3fdc18d700" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_2dacb297-8a28-46c2-b995-2fa28047a400" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_2dacb297-8a28-46c2-b995-2fa28047a400_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_2dacb297-8a28-46c2-b995-2fa28047a400" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_2dacb297-8a28-46c2-b995-2fa28047a400_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_b75c1bfe-af4e-4dda-84e2-4592d84f2035" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_2dacb297-8a28-46c2-b995-2fa28047a400" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_b75c1bfe-af4e-4dda-84e2-4592d84f2035" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TradeNamesMember_011b2e4c-1875-4a95-9906-8b630916a36a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_TradeNamesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_b75c1bfe-af4e-4dda-84e2-4592d84f2035" xlink:to="loc_us-gaap_TradeNamesMember_011b2e4c-1875-4a95-9906-8b630916a36a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoncompeteAgreementsMember_21e3350a-5f2c-469b-8c7d-969a4858394d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_NoncompeteAgreementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_b75c1bfe-af4e-4dda-84e2-4592d84f2035" xlink:to="loc_us-gaap_NoncompeteAgreementsMember_21e3350a-5f2c-469b-8c7d-969a4858394d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_121d3b00-3672-4c54-bcf9-cd2d1f7d9b0f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CustomerRelationshipsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_b75c1bfe-af4e-4dda-84e2-4592d84f2035" xlink:to="loc_us-gaap_CustomerRelationshipsMember_121d3b00-3672-4c54-bcf9-cd2d1f7d9b0f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferredAbstract_b794357f-24a9-4c48-a6a7-032278487f29" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_BusinessCombinationConsiderationTransferredAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_94f83b50-0f5f-40b4-a1ed-cc8dfb197922" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferredAbstract_b794357f-24a9-4c48-a6a7-032278487f29" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferred1_ad597c57-f9ae-4754-8dbe-8a9c313062be" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_BusinessCombinationConsiderationTransferred1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationConsiderationTransferredAbstract_b794357f-24a9-4c48-a6a7-032278487f29" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferred1_ad597c57-f9ae-4754-8dbe-8a9c313062be" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAcquiredFromAcquisition_d8a42583-280f-4c43-81b0-6b71a8a98ff4" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CashAcquiredFromAcquisition"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationConsiderationTransferredAbstract_b794357f-24a9-4c48-a6a7-032278487f29" xlink:to="loc_us-gaap_CashAcquiredFromAcquisition_d8a42583-280f-4c43-81b0-6b71a8a98ff4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNetAbstract_5b929cab-cc0d-4580-8e0c-a97a229e0958" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNetAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_94f83b50-0f5f-40b4-a1ed-cc8dfb197922" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNetAbstract_5b929cab-cc0d-4580-8e0c-a97a229e0958" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssets_d1f5bc97-5db8-4bf6-b17a-81ed02ff8209" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNetAbstract_5b929cab-cc0d-4580-8e0c-a97a229e0958" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssets_d1f5bc97-5db8-4bf6-b17a-81ed02ff8209" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLand_0b6c222a-82ff-4545-a260-720b1cded604" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLand"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNetAbstract_5b929cab-cc0d-4580-8e0c-a97a229e0958" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLand_0b6c222a-82ff-4545-a260-720b1cded604" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedBuildings_cb8bf35a-fe7b-47ce-ab7c-12967eaadc0a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedBuildings"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNetAbstract_5b929cab-cc0d-4580-8e0c-a97a229e0958" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedBuildings_cb8bf35a-fe7b-47ce-ab7c-12967eaadc0a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedEquipment_903ec2fe-8a15-43b6-908c-9d83af194b9f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNetAbstract_5b929cab-cc0d-4580-8e0c-a97a229e0958" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedEquipment_903ec2fe-8a15-43b6-908c-9d83af194b9f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeaseRightOfUseAsset_1e31e168-820c-4ee0-88ad-5f9e3ea8deeb" xlink:href="cwst-20250331.xsd#cwst_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeaseRightOfUseAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNetAbstract_5b929cab-cc0d-4580-8e0c-a97a229e0958" xlink:to="loc_cwst_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeaseRightOfUseAsset_1e31e168-820c-4ee0-88ad-5f9e3ea8deeb" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill_5e0c0fa9-9a8f-4c1d-aff2-5a3cb7ad6741" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNetAbstract_5b929cab-cc0d-4580-8e0c-a97a229e0958" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill_5e0c0fa9-9a8f-4c1d-aff2-5a3cb7ad6741" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilities_46f15949-b4fa-41d7-a0e8-586ca6cc0c4e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNetAbstract_5b929cab-cc0d-4580-8e0c-a97a229e0958" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilities_46f15949-b4fa-41d7-a0e8-586ca6cc0c4e" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeaseLiabilityNoncurrent_d6541355-8b51-4e12-9704-8880ecfb6ac6" xlink:href="cwst-20250331.xsd#cwst_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeaseLiabilityNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNetAbstract_5b929cab-cc0d-4580-8e0c-a97a229e0958" xlink:to="loc_cwst_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeaseLiabilityNoncurrent_d6541355-8b51-4e12-9704-8880ecfb6ac6" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet_4de889b7-d472-4709-9691-84f6b2700b61" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNetAbstract_5b929cab-cc0d-4580-8e0c-a97a229e0958" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet_4de889b7-d472-4709-9691-84f6b2700b61" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_af4910ea-6185-4212-88c6-af48ff501c6e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_94f83b50-0f5f-40b4-a1ed-cc8dfb197922" xlink:to="loc_us-gaap_Goodwill_af4910ea-6185-4212-88c6-af48ff501c6e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables_6b881a6b-1866-4646-8fcb-4255111f81ee" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_94f83b50-0f5f-40b4-a1ed-cc8dfb197922" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables_6b881a6b-1866-4646-8fcb-4255111f81ee" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/GOODWILLANDINTANGIBLEASSETSSummaryofGoodwillActivityDetail" xlink:type="simple" xlink:href="cwst-20250331.xsd#GOODWILLANDINTANGIBLEASSETSSummaryofGoodwillActivityDetail"/>
  <link:definitionLink xlink:role="http://casella.com/role/GOODWILLANDINTANGIBLEASSETSSummaryofGoodwillActivityDetail" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillLineItems_00f929a6-f3f6-452c-9b24-ae2915a417df" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_GoodwillLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfGoodwillTable_47821e79-2f33-41d2-b3f8-cad7e8a3529f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfGoodwillTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_GoodwillLineItems_00f929a6-f3f6-452c-9b24-ae2915a417df" xlink:to="loc_us-gaap_ScheduleOfGoodwillTable_47821e79-2f33-41d2-b3f8-cad7e8a3529f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_ed25ff56-b8f7-48e0-a07c-a7e577cc3513" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfGoodwillTable_47821e79-2f33-41d2-b3f8-cad7e8a3529f" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_ed25ff56-b8f7-48e0-a07c-a7e577cc3513" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_ed25ff56-b8f7-48e0-a07c-a7e577cc3513_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_ed25ff56-b8f7-48e0-a07c-a7e577cc3513" xlink:to="loc_us-gaap_SegmentDomain_ed25ff56-b8f7-48e0-a07c-a7e577cc3513_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_3e917df8-6303-4a06-aa3d-0b3aa6b696fd" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_ed25ff56-b8f7-48e0-a07c-a7e577cc3513" xlink:to="loc_us-gaap_SegmentDomain_3e917df8-6303-4a06-aa3d-0b3aa6b696fd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_EasternRegionMember_374a8650-c90e-4e50-b8f7-855e577b95fd" xlink:href="cwst-20250331.xsd#cwst_EasternRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_3e917df8-6303-4a06-aa3d-0b3aa6b696fd" xlink:to="loc_cwst_EasternRegionMember_374a8650-c90e-4e50-b8f7-855e577b95fd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_WesternRegionMember_77aa9def-7d51-46bf-a6d5-4277d245551e" xlink:href="cwst-20250331.xsd#cwst_WesternRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_3e917df8-6303-4a06-aa3d-0b3aa6b696fd" xlink:to="loc_cwst_WesternRegionMember_77aa9def-7d51-46bf-a6d5-4277d245551e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_MidAtlanticRegionMember_c3070b52-7b39-45a5-9715-cb8e364fe384" xlink:href="cwst-20250331.xsd#cwst_MidAtlanticRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_3e917df8-6303-4a06-aa3d-0b3aa6b696fd" xlink:to="loc_cwst_MidAtlanticRegionMember_c3070b52-7b39-45a5-9715-cb8e364fe384" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_ResourceSolutionsRegionMember_698c1835-62d7-45ef-b18a-386684a17f6f" xlink:href="cwst-20250331.xsd#cwst_ResourceSolutionsRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_3e917df8-6303-4a06-aa3d-0b3aa6b696fd" xlink:to="loc_cwst_ResourceSolutionsRegionMember_698c1835-62d7-45ef-b18a-386684a17f6f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillRollForward_b5fd3041-6727-4093-90f9-78193014c225" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_GoodwillRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillLineItems_00f929a6-f3f6-452c-9b24-ae2915a417df" xlink:to="loc_us-gaap_GoodwillRollForward_b5fd3041-6727-4093-90f9-78193014c225" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_e6c01a57-b441-4954-bef2-4a77694cd376" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillRollForward_b5fd3041-6727-4093-90f9-78193014c225" xlink:to="loc_us-gaap_Goodwill_e6c01a57-b441-4954-bef2-4a77694cd376" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillAcquiredDuringPeriod_883d03dd-4cdd-4146-ba43-759d8ec2961a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_GoodwillAcquiredDuringPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillRollForward_b5fd3041-6727-4093-90f9-78193014c225" xlink:to="loc_us-gaap_GoodwillAcquiredDuringPeriod_883d03dd-4cdd-4146-ba43-759d8ec2961a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillPurchaseAccountingAdjustments_b7ea01c0-5cee-49fc-a197-78caf368dd4f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_GoodwillPurchaseAccountingAdjustments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillRollForward_b5fd3041-6727-4093-90f9-78193014c225" xlink:to="loc_us-gaap_GoodwillPurchaseAccountingAdjustments_b7ea01c0-5cee-49fc-a197-78caf368dd4f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_5bab8b87-57b7-4add-b33f-28591f28d9cc" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_Goodwill"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/GOODWILLANDINTANGIBLEASSETSSummaryofIntangibleAssetsbyTypeDetail" xlink:type="simple" xlink:href="cwst-20250331.xsd#GOODWILLANDINTANGIBLEASSETSSummaryofIntangibleAssetsbyTypeDetail"/>
  <link:definitionLink xlink:role="http://casella.com/role/GOODWILLANDINTANGIBLEASSETSSummaryofIntangibleAssetsbyTypeDetail" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_bde32a9a-883e-449e-9b53-03551a23eb0c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FiniteLivedIntangibleAssetsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_8746f094-cbb1-41b4-a784-ad60df713bcc" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_bde32a9a-883e-449e-9b53-03551a23eb0c" xlink:to="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_8746f094-cbb1-41b4-a784-ad60df713bcc" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_d6261e3b-f1d9-4070-a86a-ce91f60047b5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_8746f094-cbb1-41b4-a784-ad60df713bcc" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_d6261e3b-f1d9-4070-a86a-ce91f60047b5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_d6261e3b-f1d9-4070-a86a-ce91f60047b5_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_d6261e3b-f1d9-4070-a86a-ce91f60047b5" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_d6261e3b-f1d9-4070-a86a-ce91f60047b5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_e4891bd5-3e9f-4737-adfd-1f7ebe4a169a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_d6261e3b-f1d9-4070-a86a-ce91f60047b5" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_e4891bd5-3e9f-4737-adfd-1f7ebe4a169a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoncompeteAgreementsMember_b2c67ada-2989-4d40-9c57-889e046eb513" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_NoncompeteAgreementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_e4891bd5-3e9f-4737-adfd-1f7ebe4a169a" xlink:to="loc_us-gaap_NoncompeteAgreementsMember_b2c67ada-2989-4d40-9c57-889e046eb513" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_5b331ce5-c170-401d-9b3c-60eccd6fea68" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CustomerRelationshipsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_e4891bd5-3e9f-4737-adfd-1f7ebe4a169a" xlink:to="loc_us-gaap_CustomerRelationshipsMember_5b331ce5-c170-401d-9b3c-60eccd6fea68" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TradeNamesMember_6fba46af-f4ab-4488-85ea-333a5e86b7bb" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_TradeNamesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_e4891bd5-3e9f-4737-adfd-1f7ebe4a169a" xlink:to="loc_us-gaap_TradeNamesMember_6fba46af-f4ab-4488-85ea-333a5e86b7bb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsGross_15734775-3e2f-4be5-bf01-e80dab8e3c35" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FiniteLivedIntangibleAssetsGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_bde32a9a-883e-449e-9b53-03551a23eb0c" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsGross_15734775-3e2f-4be5-bf01-e80dab8e3c35" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_d97708c8-b129-4212-82aa-5e49e52b924b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_bde32a9a-883e-449e-9b53-03551a23eb0c" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_d97708c8-b129-4212-82aa-5e49e52b924b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsNet_b79e8dff-bd95-452a-a3dc-13a15bbbf590" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FiniteLivedIntangibleAssetsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_bde32a9a-883e-449e-9b53-03551a23eb0c" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsNet_b79e8dff-bd95-452a-a3dc-13a15bbbf590" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/DEBTSummaryofDebtDetails" xlink:type="simple" xlink:href="cwst-20250331.xsd#DEBTSummaryofDebtDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/DEBTSummaryofDebtDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_91e1ca23-e57c-4e7b-ab3b-e39be4746a5e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_ed670374-5913-4b9a-8107-fb22eb48d7de" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_91e1ca23-e57c-4e7b-ab3b-e39be4746a5e" xlink:to="loc_us-gaap_DebtInstrumentTable_ed670374-5913-4b9a-8107-fb22eb48d7de" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_a7a16066-0ab6-4679-8188-fdd02ba3dc44" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_ed670374-5913-4b9a-8107-fb22eb48d7de" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_a7a16066-0ab6-4679-8188-fdd02ba3dc44" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_a7a16066-0ab6-4679-8188-fdd02ba3dc44_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_a7a16066-0ab6-4679-8188-fdd02ba3dc44" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_a7a16066-0ab6-4679-8188-fdd02ba3dc44_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_1257db30-baf6-4b8c-b375-f2c3805c7352" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_a7a16066-0ab6-4679-8188-fdd02ba3dc44" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_1257db30-baf6-4b8c-b375-f2c3805c7352" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_5898705a-cf1d-45e7-8403-7d3e8cd52703" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_1257db30-baf6-4b8c-b375-f2c3805c7352" xlink:to="loc_us-gaap_SecuredDebtMember_5898705a-cf1d-45e7-8403-7d3e8cd52703" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_1e0b7fa3-4ab2-4e19-97d3-37715fa210d7" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_1257db30-baf6-4b8c-b375-f2c3805c7352" xlink:to="loc_us-gaap_LineOfCreditMember_1e0b7fa3-4ab2-4e19-97d3-37715fa210d7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnsecuredDebtMember_accc1940-320a-41ca-b882-64a6ad845ceb" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_UnsecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_1257db30-baf6-4b8c-b375-f2c3805c7352" xlink:to="loc_us-gaap_UnsecuredDebtMember_accc1940-320a-41ca-b882-64a6ad845ceb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_NotesPayableMember_71ca1bf7-d64c-4a10-894b-a3ca2ee04400" xlink:href="cwst-20250331.xsd#cwst_NotesPayableMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_1257db30-baf6-4b8c-b375-f2c3805c7352" xlink:to="loc_cwst_NotesPayableMember_71ca1bf7-d64c-4a10-894b-a3ca2ee04400" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_929d562b-3c21-4e5d-b1cf-21a91de373f2" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_ed670374-5913-4b9a-8107-fb22eb48d7de" xlink:to="loc_us-gaap_DebtInstrumentAxis_929d562b-3c21-4e5d-b1cf-21a91de373f2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_929d562b-3c21-4e5d-b1cf-21a91de373f2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_929d562b-3c21-4e5d-b1cf-21a91de373f2" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_929d562b-3c21-4e5d-b1cf-21a91de373f2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_4b25fec8-096b-448f-a639-091ea1c8a84a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_929d562b-3c21-4e5d-b1cf-21a91de373f2" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_4b25fec8-096b-448f-a639-091ea1c8a84a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_TermLoanAFacilityDueSeptember2029Member_6092cfef-b5a9-4c30-a3a3-604ba4dc6bc3" xlink:href="cwst-20250331.xsd#cwst_TermLoanAFacilityDueSeptember2029Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_4b25fec8-096b-448f-a639-091ea1c8a84a" xlink:to="loc_cwst_TermLoanAFacilityDueSeptember2029Member_6092cfef-b5a9-4c30-a3a3-604ba4dc6bc3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_RevolvingCreditFacilityDueSeptember2029Member_7ce08be7-c1c8-4648-9c28-354e973d543a" xlink:href="cwst-20250331.xsd#cwst_RevolvingCreditFacilityDueSeptember2029Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_4b25fec8-096b-448f-a639-091ea1c8a84a" xlink:to="loc_cwst_RevolvingCreditFacilityDueSeptember2029Member_7ce08be7-c1c8-4648-9c28-354e973d543a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_RevenueBondsSeriesTwoThousandFourteenDueTwoThousandFortyFourMember_ab6a2bb9-eb07-42a6-bc6f-a2a0257ea2d4" xlink:href="cwst-20250331.xsd#cwst_RevenueBondsSeriesTwoThousandFourteenDueTwoThousandFortyFourMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_4b25fec8-096b-448f-a639-091ea1c8a84a" xlink:to="loc_cwst_RevenueBondsSeriesTwoThousandFourteenDueTwoThousandFortyFourMember_ab6a2bb9-eb07-42a6-bc6f-a2a0257ea2d4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_RevenueBondSeriesTwoThousandFourteenRTwoDueTwoThousandFortyFourMember_cad8398d-0dd2-446d-80f8-bd2df4d97115" xlink:href="cwst-20250331.xsd#cwst_RevenueBondSeriesTwoThousandFourteenRTwoDueTwoThousandFortyFourMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_4b25fec8-096b-448f-a639-091ea1c8a84a" xlink:to="loc_cwst_RevenueBondSeriesTwoThousandFourteenRTwoDueTwoThousandFortyFourMember_cad8398d-0dd2-446d-80f8-bd2df4d97115" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_RevenueBondsSeries2020DueSeptember2050Member_cb90754b-8c38-432e-a0e1-5f67f6753385" xlink:href="cwst-20250331.xsd#cwst_RevenueBondsSeries2020DueSeptember2050Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_4b25fec8-096b-448f-a639-091ea1c8a84a" xlink:to="loc_cwst_RevenueBondsSeries2020DueSeptember2050Member_cb90754b-8c38-432e-a0e1-5f67f6753385" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_RevenueBondsSeries2020R2DueSeptember2050Member_c6c8545b-4208-4143-9310-a4f341e7e509" xlink:href="cwst-20250331.xsd#cwst_RevenueBondsSeries2020R2DueSeptember2050Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_4b25fec8-096b-448f-a639-091ea1c8a84a" xlink:to="loc_cwst_RevenueBondsSeries2020R2DueSeptember2050Member_c6c8545b-4208-4143-9310-a4f341e7e509" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_FinanceAuthorityofMaineSolidWasteDisposalRevenueBondsSeries2005R3FAMEBonds2005R3Member_4f89a9b3-0b66-45bb-9c94-50406632fb5c" xlink:href="cwst-20250331.xsd#cwst_FinanceAuthorityofMaineSolidWasteDisposalRevenueBondsSeries2005R3FAMEBonds2005R3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_4b25fec8-096b-448f-a639-091ea1c8a84a" xlink:to="loc_cwst_FinanceAuthorityofMaineSolidWasteDisposalRevenueBondsSeries2005R3FAMEBonds2005R3Member_4f89a9b3-0b66-45bb-9c94-50406632fb5c" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_FinanceAuthorityOfMaineSolidWasteDisposalRevenueBondsSeries2015R1Member_8d4553ec-0fe9-43f4-bb79-39c2c3708285" xlink:href="cwst-20250331.xsd#cwst_FinanceAuthorityOfMaineSolidWasteDisposalRevenueBondsSeries2015R1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_4b25fec8-096b-448f-a639-091ea1c8a84a" xlink:to="loc_cwst_FinanceAuthorityOfMaineSolidWasteDisposalRevenueBondsSeries2015R1Member_8d4553ec-0fe9-43f4-bb79-39c2c3708285" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_FinanceAuthorityOfMaineSolidWasteDisposalRevenueBondsSeries2015R2Member_5231c0fc-656e-4cfd-96f5-2e68ba6f04dd" xlink:href="cwst-20250331.xsd#cwst_FinanceAuthorityOfMaineSolidWasteDisposalRevenueBondsSeries2015R2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_4b25fec8-096b-448f-a639-091ea1c8a84a" xlink:to="loc_cwst_FinanceAuthorityOfMaineSolidWasteDisposalRevenueBondsSeries2015R2Member_5231c0fc-656e-4cfd-96f5-2e68ba6f04dd" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_FinanceAuthorityOfMaineSolidWasteDisposalRevenueBondsSeries2024Member_539d49b5-4d0c-4730-9aea-c5b9589ce22b" xlink:href="cwst-20250331.xsd#cwst_FinanceAuthorityOfMaineSolidWasteDisposalRevenueBondsSeries2024Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_4b25fec8-096b-448f-a639-091ea1c8a84a" xlink:to="loc_cwst_FinanceAuthorityOfMaineSolidWasteDisposalRevenueBondsSeries2024Member_539d49b5-4d0c-4730-9aea-c5b9589ce22b" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_LongTermRevenueBondSeriesTwoThousandThirteenDueTwoThousandThirtySixMember_553cdd70-1c36-4e75-840e-53bdbaa4b160" xlink:href="cwst-20250331.xsd#cwst_LongTermRevenueBondSeriesTwoThousandThirteenDueTwoThousandThirtySixMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_4b25fec8-096b-448f-a639-091ea1c8a84a" xlink:to="loc_cwst_LongTermRevenueBondSeriesTwoThousandThirteenDueTwoThousandThirtySixMember_553cdd70-1c36-4e75-840e-53bdbaa4b160" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_VermontBondsSeries2022A1Due2052Member_98b1366f-60eb-4612-9d94-13eac75b6aff" xlink:href="cwst-20250331.xsd#cwst_VermontBondsSeries2022A1Due2052Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_4b25fec8-096b-448f-a639-091ea1c8a84a" xlink:to="loc_cwst_VermontBondsSeries2022A1Due2052Member_98b1366f-60eb-4612-9d94-13eac75b6aff" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_VermontBondsSeries2022A2Due2052Member_d71a3404-85fb-4c02-b995-2140e6b61a5f" xlink:href="cwst-20250331.xsd#cwst_VermontBondsSeries2022A2Due2052Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_4b25fec8-096b-448f-a639-091ea1c8a84a" xlink:to="loc_cwst_VermontBondsSeries2022A2Due2052Member_d71a3404-85fb-4c02-b995-2140e6b61a5f" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_RevenueBondSeriesTwoThousandThirteenDueTwoThousandTwentyNineMember_7c0e6251-5e43-4f07-9204-875455d07c53" xlink:href="cwst-20250331.xsd#cwst_RevenueBondSeriesTwoThousandThirteenDueTwoThousandTwentyNineMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_4b25fec8-096b-448f-a639-091ea1c8a84a" xlink:to="loc_cwst_RevenueBondSeriesTwoThousandThirteenDueTwoThousandTwentyNineMember_7c0e6251-5e43-4f07-9204-875455d07c53" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_522456f9-955e-49f6-b690-ae80604fe8d8" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_91e1ca23-e57c-4e7b-ab3b-e39be4746a5e" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_522456f9-955e-49f6-b690-ae80604fe8d8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_074e8115-d0d7-42a2-bf46-0efc25bc8721" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_91e1ca23-e57c-4e7b-ab3b-e39be4746a5e" xlink:to="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_074e8115-d0d7-42a2-bf46-0efc25bc8721" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinanceLeaseWeightedAverageDiscountRatePercent_761f094d-67ad-4b89-b39f-8a310cf1cf68" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FinanceLeaseWeightedAverageDiscountRatePercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_91e1ca23-e57c-4e7b-ab3b-e39be4746a5e" xlink:to="loc_us-gaap_FinanceLeaseWeightedAverageDiscountRatePercent_761f094d-67ad-4b89-b39f-8a310cf1cf68" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentCarryingAmount_4022abaf-85e8-40d4-a0c2-f6852c8c4aa9" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_91e1ca23-e57c-4e7b-ab3b-e39be4746a5e" xlink:to="loc_us-gaap_DebtInstrumentCarryingAmount_4022abaf-85e8-40d4-a0c2-f6852c8c4aa9" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinanceLeaseLiability_b1cccad6-d51c-4519-abf6-72fb4933f764" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FinanceLeaseLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_91e1ca23-e57c-4e7b-ab3b-e39be4746a5e" xlink:to="loc_us-gaap_FinanceLeaseLiability_b1cccad6-d51c-4519-abf6-72fb4933f764" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinanceLeaseLiabilityStatementOfFinancialPositionExtensibleList_863a39ac-04b6-40f4-a9c6-c7d9058ea91d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FinanceLeaseLiabilityStatementOfFinancialPositionExtensibleList"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinanceLeaseLiability_b1cccad6-d51c-4519-abf6-72fb4933f764" xlink:to="loc_us-gaap_FinanceLeaseLiabilityStatementOfFinancialPositionExtensibleList_863a39ac-04b6-40f4-a9c6-c7d9058ea91d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtAndCapitalLeaseObligations_4c1c25b6-0b81-438d-acdf-76162d9446b6" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtAndCapitalLeaseObligations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_91e1ca23-e57c-4e7b-ab3b-e39be4746a5e" xlink:to="loc_us-gaap_DebtAndCapitalLeaseObligations_4c1c25b6-0b81-438d-acdf-76162d9446b6" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredFinanceCostsNet_e805f9a1-f998-4ede-8592-c5dd7ab9bbfb" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DeferredFinanceCostsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_91e1ca23-e57c-4e7b-ab3b-e39be4746a5e" xlink:to="loc_us-gaap_DeferredFinanceCostsNet_e805f9a1-f998-4ede-8592-c5dd7ab9bbfb" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtAndCapitalLeaseObligationsIncludingCurrentMaturities_779007df-77c2-4c1b-8202-b68510caecf4" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LongTermDebtAndCapitalLeaseObligationsIncludingCurrentMaturities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_91e1ca23-e57c-4e7b-ab3b-e39be4746a5e" xlink:to="loc_us-gaap_LongTermDebtAndCapitalLeaseObligationsIncludingCurrentMaturities_779007df-77c2-4c1b-8202-b68510caecf4" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtAndCapitalLeaseObligationsCurrent_02d01e41-8801-4666-a3df-688f8f680071" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LongTermDebtAndCapitalLeaseObligationsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_91e1ca23-e57c-4e7b-ab3b-e39be4746a5e" xlink:to="loc_us-gaap_LongTermDebtAndCapitalLeaseObligationsCurrent_02d01e41-8801-4666-a3df-688f8f680071" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtAndCapitalLeaseObligations_459c9ae2-f06f-47ac-8e0c-7da0a4cc6f68" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LongTermDebtAndCapitalLeaseObligations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_91e1ca23-e57c-4e7b-ab3b-e39be4746a5e" xlink:to="loc_us-gaap_LongTermDebtAndCapitalLeaseObligations_459c9ae2-f06f-47ac-8e0c-7da0a4cc6f68" xlink:type="arc" order="9"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/DEBTNarrativeCreditFacilityDetails" xlink:type="simple" xlink:href="cwst-20250331.xsd#DEBTNarrativeCreditFacilityDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/DEBTNarrativeCreditFacilityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityLineItems_0a9a3cd3-a8cd-461a-bf34-06283bf990cc" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LineOfCreditFacilityLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityTable_8315c9cd-6d59-4577-8149-5d3e432a2199" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LineOfCreditFacilityTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_0a9a3cd3-a8cd-461a-bf34-06283bf990cc" xlink:to="loc_us-gaap_LineOfCreditFacilityTable_8315c9cd-6d59-4577-8149-5d3e432a2199" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_8b1723dc-7bab-4e40-ba14-2f362e717314" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_8315c9cd-6d59-4577-8149-5d3e432a2199" xlink:to="loc_us-gaap_DebtInstrumentAxis_8b1723dc-7bab-4e40-ba14-2f362e717314" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_8b1723dc-7bab-4e40-ba14-2f362e717314_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_8b1723dc-7bab-4e40-ba14-2f362e717314" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_8b1723dc-7bab-4e40-ba14-2f362e717314_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_2fc8f7f7-8fa7-4b97-8ef8-0c43aa8d9859" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_8b1723dc-7bab-4e40-ba14-2f362e717314" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_2fc8f7f7-8fa7-4b97-8ef8-0c43aa8d9859" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_CreditFacilityMember_bf02060c-236f-4591-bf03-74c8476dfbcd" xlink:href="cwst-20250331.xsd#cwst_CreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_2fc8f7f7-8fa7-4b97-8ef8-0c43aa8d9859" xlink:to="loc_cwst_CreditFacilityMember_bf02060c-236f-4591-bf03-74c8476dfbcd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_21b07fbc-69b3-4fb5-973e-b6cfbefabe03" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_8315c9cd-6d59-4577-8149-5d3e432a2199" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_21b07fbc-69b3-4fb5-973e-b6cfbefabe03" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_21b07fbc-69b3-4fb5-973e-b6cfbefabe03_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_21b07fbc-69b3-4fb5-973e-b6cfbefabe03" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_21b07fbc-69b3-4fb5-973e-b6cfbefabe03_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_0d6ed41f-35d1-4287-85be-17afdb309fdb" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_21b07fbc-69b3-4fb5-973e-b6cfbefabe03" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_0d6ed41f-35d1-4287-85be-17afdb309fdb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_916b4937-0900-484e-9953-24c036f28595" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_0d6ed41f-35d1-4287-85be-17afdb309fdb" xlink:to="loc_us-gaap_SecuredDebtMember_916b4937-0900-484e-9953-24c036f28595" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_acc04444-3e24-4911-9c8a-19c5b269328d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_0d6ed41f-35d1-4287-85be-17afdb309fdb" xlink:to="loc_us-gaap_LineOfCreditMember_acc04444-3e24-4911-9c8a-19c5b269328d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_f4dfa0f8-0c91-4be1-9744-b5087b81a369" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_8315c9cd-6d59-4577-8149-5d3e432a2199" xlink:to="loc_srt_RangeAxis_f4dfa0f8-0c91-4be1-9744-b5087b81a369" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_f4dfa0f8-0c91-4be1-9744-b5087b81a369_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_f4dfa0f8-0c91-4be1-9744-b5087b81a369" xlink:to="loc_srt_RangeMember_f4dfa0f8-0c91-4be1-9744-b5087b81a369_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_a5362f2e-9c6e-409b-a22b-794a47e76947" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_f4dfa0f8-0c91-4be1-9744-b5087b81a369" xlink:to="loc_srt_RangeMember_a5362f2e-9c6e-409b-a22b-794a47e76947" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_bf44e15f-0035-44b2-a630-5efd6ed9e67e" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_a5362f2e-9c6e-409b-a22b-794a47e76947" xlink:to="loc_srt_MinimumMember_bf44e15f-0035-44b2-a630-5efd6ed9e67e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_48fd3a80-858a-40d9-ba52-e1502757e1d0" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_a5362f2e-9c6e-409b-a22b-794a47e76947" xlink:to="loc_srt_MaximumMember_48fd3a80-858a-40d9-ba52-e1502757e1d0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_c38045b8-ce15-4d27-ad62-fc75d7e7cd2e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_8315c9cd-6d59-4577-8149-5d3e432a2199" xlink:to="loc_us-gaap_CreditFacilityAxis_c38045b8-ce15-4d27-ad62-fc75d7e7cd2e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_c38045b8-ce15-4d27-ad62-fc75d7e7cd2e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_c38045b8-ce15-4d27-ad62-fc75d7e7cd2e" xlink:to="loc_us-gaap_CreditFacilityDomain_c38045b8-ce15-4d27-ad62-fc75d7e7cd2e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_929b24cf-63c3-434e-9221-4cac3eee6665" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_c38045b8-ce15-4d27-ad62-fc75d7e7cd2e" xlink:to="loc_us-gaap_CreditFacilityDomain_929b24cf-63c3-434e-9221-4cac3eee6665" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_2ea0b409-61a8-4e5e-9700-2ee83b012065" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_929b24cf-63c3-434e-9221-4cac3eee6665" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_2ea0b409-61a8-4e5e-9700-2ee83b012065" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LetterOfCreditMember_9569879d-81d4-4292-8dc9-8f2f7fd5a7ec" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LetterOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_929b24cf-63c3-434e-9221-4cac3eee6665" xlink:to="loc_us-gaap_LetterOfCreditMember_9569879d-81d4-4292-8dc9-8f2f7fd5a7ec" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_b16ebc74-6776-45ed-8276-28c6fdbc896d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_0a9a3cd3-a8cd-461a-bf34-06283bf990cc" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_b16ebc74-6776-45ed-8276-28c6fdbc896d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_3f884979-17c8-4d48-982e-e6560139bbe9" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_0a9a3cd3-a8cd-461a-bf34-06283bf990cc" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_3f884979-17c8-4d48-982e-e6560139bbe9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_LettersOfCreditSublimit_609cafcc-a319-40af-b0d4-0e31282a8f7c" xlink:href="cwst-20250331.xsd#cwst_LettersOfCreditSublimit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_0a9a3cd3-a8cd-461a-bf34-06283bf990cc" xlink:to="loc_cwst_LettersOfCreditSublimit_609cafcc-a319-40af-b0d4-0e31282a8f7c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_LineOfCreditFacilityAdditionalBorrowingCapacity_d1e7c45f-9a04-4a2b-84d8-6c492dcc1fda" xlink:href="cwst-20250331.xsd#cwst_LineOfCreditFacilityAdditionalBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_0a9a3cd3-a8cd-461a-bf34-06283bf990cc" xlink:to="loc_cwst_LineOfCreditFacilityAdditionalBorrowingCapacity_d1e7c45f-9a04-4a2b-84d8-6c492dcc1fda" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTerm_14b873c7-25ce-490d-be40-b07c39404a87" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_0a9a3cd3-a8cd-461a-bf34-06283bf990cc" xlink:to="loc_us-gaap_DebtInstrumentTerm_14b873c7-25ce-490d-be40-b07c39404a87" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_DebtInstrumentVariableRateAdjustment_b0ebca9a-92c3-4f2c-aff1-ce33fa2dc02d" xlink:href="cwst-20250331.xsd#cwst_DebtInstrumentVariableRateAdjustment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_0a9a3cd3-a8cd-461a-bf34-06283bf990cc" xlink:to="loc_cwst_DebtInstrumentVariableRateAdjustment_b0ebca9a-92c3-4f2c-aff1-ce33fa2dc02d" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage_00e85dc5-45a7-4e68-9f2b-a2635c738407" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_0a9a3cd3-a8cd-461a-bf34-06283bf990cc" xlink:to="loc_us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage_00e85dc5-45a7-4e68-9f2b-a2635c738407" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentageAdjustment_c0d8c869-960f-43f0-9f49-f7b542d72a21" xlink:href="cwst-20250331.xsd#cwst_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentageAdjustment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_0a9a3cd3-a8cd-461a-bf34-06283bf990cc" xlink:to="loc_cwst_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentageAdjustment_c0d8c869-960f-43f0-9f49-f7b542d72a21" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_DebtInstrumentFloorInterestRate_83a09942-7873-4c9e-a177-24ba15a93008" xlink:href="cwst-20250331.xsd#cwst_DebtInstrumentFloorInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_0a9a3cd3-a8cd-461a-bf34-06283bf990cc" xlink:to="loc_cwst_DebtInstrumentFloorInterestRate_83a09942-7873-4c9e-a177-24ba15a93008" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_LineOfCreditFacilityFrontingFeePercentage_5c3df8b1-78c3-432c-bc06-8eadf5e9d9eb" xlink:href="cwst-20250331.xsd#cwst_LineOfCreditFacilityFrontingFeePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_0a9a3cd3-a8cd-461a-bf34-06283bf990cc" xlink:to="loc_cwst_LineOfCreditFacilityFrontingFeePercentage_5c3df8b1-78c3-432c-bc06-8eadf5e9d9eb" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_DebtInstrumentInterestRateAnnualIncreaseUponPaymentDefault_d4305071-d308-4fd1-88e3-48e83184dbcd" xlink:href="cwst-20250331.xsd#cwst_DebtInstrumentInterestRateAnnualIncreaseUponPaymentDefault"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_0a9a3cd3-a8cd-461a-bf34-06283bf990cc" xlink:to="loc_cwst_DebtInstrumentInterestRateAnnualIncreaseUponPaymentDefault_d4305071-d308-4fd1-88e3-48e83184dbcd" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_DebtInstrumentInterestRateAnnualIncreaseUponOtherEventOfDefault_66b88a69-25e2-47e9-85a6-93ed3091df99" xlink:href="cwst-20250331.xsd#cwst_DebtInstrumentInterestRateAnnualIncreaseUponOtherEventOfDefault"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_0a9a3cd3-a8cd-461a-bf34-06283bf990cc" xlink:to="loc_cwst_DebtInstrumentInterestRateAnnualIncreaseUponOtherEventOfDefault_66b88a69-25e2-47e9-85a6-93ed3091df99" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity_c391b990-834e-406d-889f-d9adcac7ecc2" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_0a9a3cd3-a8cd-461a-bf34-06283bf990cc" xlink:to="loc_us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity_c391b990-834e-406d-889f-d9adcac7ecc2" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LettersOfCreditOutstandingAmount_3a69b20b-46e3-4c40-b66b-4fc3da2ea852" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LettersOfCreditOutstandingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_0a9a3cd3-a8cd-461a-bf34-06283bf990cc" xlink:to="loc_us-gaap_LettersOfCreditOutstandingAmount_3a69b20b-46e3-4c40-b66b-4fc3da2ea852" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCredit_becc71af-4d3f-4cc3-9a6e-b5df561fb862" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LineOfCredit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_0a9a3cd3-a8cd-461a-bf34-06283bf990cc" xlink:to="loc_us-gaap_LineOfCredit_becc71af-4d3f-4cc3-9a6e-b5df561fb862" xlink:type="arc" order="14"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/DEBTScheduleofVariableRateMarginsDetails" xlink:type="simple" xlink:href="cwst-20250331.xsd#DEBTScheduleofVariableRateMarginsDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/DEBTScheduleofVariableRateMarginsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_1b8c43fc-9ee9-4b00-9638-e24591a82055" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_410da0ae-c33b-419a-ae80-9e14f179b7f4" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_1b8c43fc-9ee9-4b00-9638-e24591a82055" xlink:to="loc_us-gaap_DebtInstrumentTable_410da0ae-c33b-419a-ae80-9e14f179b7f4" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_b41c4bbf-d541-447f-8f69-4e27ac2aed07" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_410da0ae-c33b-419a-ae80-9e14f179b7f4" xlink:to="loc_us-gaap_DebtInstrumentAxis_b41c4bbf-d541-447f-8f69-4e27ac2aed07" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_b41c4bbf-d541-447f-8f69-4e27ac2aed07_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_b41c4bbf-d541-447f-8f69-4e27ac2aed07" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_b41c4bbf-d541-447f-8f69-4e27ac2aed07_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_e6061a08-59f5-4610-a435-204bab33a8e3" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_b41c4bbf-d541-447f-8f69-4e27ac2aed07" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_e6061a08-59f5-4610-a435-204bab33a8e3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_CreditFacilityMember_36d7d061-1214-4cae-9e71-d5f3c3009b43" xlink:href="cwst-20250331.xsd#cwst_CreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_e6061a08-59f5-4610-a435-204bab33a8e3" xlink:to="loc_cwst_CreditFacilityMember_36d7d061-1214-4cae-9e71-d5f3c3009b43" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_ec7ef3fd-e5eb-4a03-9742-8eea6be30917" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_410da0ae-c33b-419a-ae80-9e14f179b7f4" xlink:to="loc_srt_RangeAxis_ec7ef3fd-e5eb-4a03-9742-8eea6be30917" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_ec7ef3fd-e5eb-4a03-9742-8eea6be30917_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_ec7ef3fd-e5eb-4a03-9742-8eea6be30917" xlink:to="loc_srt_RangeMember_ec7ef3fd-e5eb-4a03-9742-8eea6be30917_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_5fe62aae-89e0-42f2-9f6a-15730d781fbd" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_ec7ef3fd-e5eb-4a03-9742-8eea6be30917" xlink:to="loc_srt_RangeMember_5fe62aae-89e0-42f2-9f6a-15730d781fbd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_49d3e270-7985-44fa-bbaf-aa686a5d48da" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_5fe62aae-89e0-42f2-9f6a-15730d781fbd" xlink:to="loc_srt_MinimumMember_49d3e270-7985-44fa-bbaf-aa686a5d48da" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_282ae1ad-553e-4acf-8695-d0dd01bd4bba" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_5fe62aae-89e0-42f2-9f6a-15730d781fbd" xlink:to="loc_srt_MaximumMember_282ae1ad-553e-4acf-8695-d0dd01bd4bba" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_d3146278-f36a-44be-91be-4166cbf82429" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_410da0ae-c33b-419a-ae80-9e14f179b7f4" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_d3146278-f36a-44be-91be-4166cbf82429" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_d3146278-f36a-44be-91be-4166cbf82429_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_d3146278-f36a-44be-91be-4166cbf82429" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_d3146278-f36a-44be-91be-4166cbf82429_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_011dc1ef-14a9-4930-ac8d-14a7b970eee9" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_d3146278-f36a-44be-91be-4166cbf82429" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_011dc1ef-14a9-4930-ac8d-14a7b970eee9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_a91d73f6-747c-45cd-945a-9fe5b66b73ed" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_011dc1ef-14a9-4930-ac8d-14a7b970eee9" xlink:to="loc_us-gaap_LineOfCreditMember_a91d73f6-747c-45cd-945a-9fe5b66b73ed" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateAxis_cdddf3fb-851e-4385-8b5e-bb5969c9dbc1" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_VariableRateAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_410da0ae-c33b-419a-ae80-9e14f179b7f4" xlink:to="loc_us-gaap_VariableRateAxis_cdddf3fb-851e-4385-8b5e-bb5969c9dbc1" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_cdddf3fb-851e-4385-8b5e-bb5969c9dbc1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_VariableRateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VariableRateAxis_cdddf3fb-851e-4385-8b5e-bb5969c9dbc1" xlink:to="loc_us-gaap_VariableRateDomain_cdddf3fb-851e-4385-8b5e-bb5969c9dbc1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_0d9321f7-c814-46fa-979d-ca1b9701b83d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_VariableRateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VariableRateAxis_cdddf3fb-851e-4385-8b5e-bb5969c9dbc1" xlink:to="loc_us-gaap_VariableRateDomain_0d9321f7-c814-46fa-979d-ca1b9701b83d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredOvernightFinancingRateSofrMember_badc5abf-b05c-4a85-b18b-a46972fb87cc" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SecuredOvernightFinancingRateSofrMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_0d9321f7-c814-46fa-979d-ca1b9701b83d" xlink:to="loc_us-gaap_SecuredOvernightFinancingRateSofrMember_badc5abf-b05c-4a85-b18b-a46972fb87cc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BaseRateMember_e10e42fd-cd9b-4e28-b657-3efb753df486" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_BaseRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_0d9321f7-c814-46fa-979d-ca1b9701b83d" xlink:to="loc_us-gaap_BaseRateMember_e10e42fd-cd9b-4e28-b657-3efb753df486" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_2d4341d9-6521-4b51-a711-1252bb936761" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_1b8c43fc-9ee9-4b00-9638-e24591a82055" xlink:to="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_2d4341d9-6521-4b51-a711-1252bb936761" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/DEBTNarrativeTaxExemptFinancingsDetails" xlink:type="simple" xlink:href="cwst-20250331.xsd#DEBTNarrativeTaxExemptFinancingsDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/DEBTNarrativeTaxExemptFinancingsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_f0922719-b3eb-4d10-b46a-c7385e2edcb7" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_ff56b327-ee7e-4143-bfc1-06a195899e07" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_f0922719-b3eb-4d10-b46a-c7385e2edcb7" xlink:to="loc_us-gaap_DebtInstrumentTable_ff56b327-ee7e-4143-bfc1-06a195899e07" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_5ed741ed-53f2-4cd5-a6c3-310441a580a8" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_ff56b327-ee7e-4143-bfc1-06a195899e07" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_5ed741ed-53f2-4cd5-a6c3-310441a580a8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_5ed741ed-53f2-4cd5-a6c3-310441a580a8_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_5ed741ed-53f2-4cd5-a6c3-310441a580a8" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_5ed741ed-53f2-4cd5-a6c3-310441a580a8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_8bd06900-afd7-473a-aaed-823b30a1f3aa" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_5ed741ed-53f2-4cd5-a6c3-310441a580a8" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_8bd06900-afd7-473a-aaed-823b30a1f3aa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnsecuredDebtMember_b8696921-a96e-401b-9980-f36c0a418f3b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_UnsecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_8bd06900-afd7-473a-aaed-823b30a1f3aa" xlink:to="loc_us-gaap_UnsecuredDebtMember_b8696921-a96e-401b-9980-f36c0a418f3b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_3a3e6025-5f5d-4518-b43b-ffa5059a8295" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_ff56b327-ee7e-4143-bfc1-06a195899e07" xlink:to="loc_us-gaap_DebtInstrumentAxis_3a3e6025-5f5d-4518-b43b-ffa5059a8295" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_3a3e6025-5f5d-4518-b43b-ffa5059a8295_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_3a3e6025-5f5d-4518-b43b-ffa5059a8295" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_3a3e6025-5f5d-4518-b43b-ffa5059a8295_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_52ef6f62-6a40-4a30-a692-771534e3d832" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_3a3e6025-5f5d-4518-b43b-ffa5059a8295" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_52ef6f62-6a40-4a30-a692-771534e3d832" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_VermontBondsSeries2022A2Due2052Member_587c3857-23df-4996-8562-e2cc718356a1" xlink:href="cwst-20250331.xsd#cwst_VermontBondsSeries2022A2Due2052Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_52ef6f62-6a40-4a30-a692-771534e3d832" xlink:to="loc_cwst_VermontBondsSeries2022A2Due2052Member_587c3857-23df-4996-8562-e2cc718356a1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_VermontBondsSeries2022A1Due2052Member_b077b44e-6d83-4b4c-83e0-796e6b6f79e8" xlink:href="cwst-20250331.xsd#cwst_VermontBondsSeries2022A1Due2052Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_52ef6f62-6a40-4a30-a692-771534e3d832" xlink:to="loc_cwst_VermontBondsSeries2022A1Due2052Member_b077b44e-6d83-4b4c-83e0-796e6b6f79e8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_FinanceAuthorityOfMaineSolidWasteDisposalRevenueBondsSeries2024Member_0582ca60-a35c-4c5e-b018-10ae8412c4cc" xlink:href="cwst-20250331.xsd#cwst_FinanceAuthorityOfMaineSolidWasteDisposalRevenueBondsSeries2024Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_52ef6f62-6a40-4a30-a692-771534e3d832" xlink:to="loc_cwst_FinanceAuthorityOfMaineSolidWasteDisposalRevenueBondsSeries2024Member_0582ca60-a35c-4c5e-b018-10ae8412c4cc" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_FinanceAuthorityOfMaineSolidWasteDisposalRevenueBondsSeries2024RestrictedMember_fe39c0ea-b4f8-41d0-a847-6fe7d6d0bd92" xlink:href="cwst-20250331.xsd#cwst_FinanceAuthorityOfMaineSolidWasteDisposalRevenueBondsSeries2024RestrictedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cwst_FinanceAuthorityOfMaineSolidWasteDisposalRevenueBondsSeries2024Member_0582ca60-a35c-4c5e-b018-10ae8412c4cc" xlink:to="loc_cwst_FinanceAuthorityOfMaineSolidWasteDisposalRevenueBondsSeries2024RestrictedMember_fe39c0ea-b4f8-41d0-a847-6fe7d6d0bd92" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_904b90d4-6cc2-4aae-9746-47c193e9791d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_f0922719-b3eb-4d10-b46a-c7385e2edcb7" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_904b90d4-6cc2-4aae-9746-47c193e9791d" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/DEBTNarrativeCashFlowHedgesDetails" xlink:type="simple" xlink:href="cwst-20250331.xsd#DEBTNarrativeCashFlowHedgesDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/DEBTNarrativeCashFlowHedgesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeLineItems_3671778a-0ae4-4cac-a2a3-203d8b992ab9" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DerivativeLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeTable_8f0f302b-8016-4e99-a09f-54334b5ad958" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DerivativeTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DerivativeLineItems_3671778a-0ae4-4cac-a2a3-203d8b992ab9" xlink:to="loc_us-gaap_DerivativeTable_8f0f302b-8016-4e99-a09f-54334b5ad958" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_6422951b-2cb7-4329-93a1-c0fc512e639c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DerivativeInstrumentRiskAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeTable_8f0f302b-8016-4e99-a09f-54334b5ad958" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_6422951b-2cb7-4329-93a1-c0fc512e639c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_6422951b-2cb7-4329-93a1-c0fc512e639c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_6422951b-2cb7-4329-93a1-c0fc512e639c" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_6422951b-2cb7-4329-93a1-c0fc512e639c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_eafd72a2-b068-4064-a01d-84a36650e383" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_6422951b-2cb7-4329-93a1-c0fc512e639c" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_eafd72a2-b068-4064-a01d-84a36650e383" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestRateContractMember_d831dc76-5734-423e-a11d-1b14de220228" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_InterestRateContractMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_eafd72a2-b068-4064-a01d-84a36650e383" xlink:to="loc_us-gaap_InterestRateContractMember_d831dc76-5734-423e-a11d-1b14de220228" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_b4e7606c-3c54-4288-9b2a-72a0f4b7e3e3" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeTable_8f0f302b-8016-4e99-a09f-54334b5ad958" xlink:to="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_b4e7606c-3c54-4288-9b2a-72a0f4b7e3e3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingRelationshipDomain_b4e7606c-3c54-4288-9b2a-72a0f4b7e3e3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_HedgingRelationshipDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_b4e7606c-3c54-4288-9b2a-72a0f4b7e3e3" xlink:to="loc_us-gaap_HedgingRelationshipDomain_b4e7606c-3c54-4288-9b2a-72a0f4b7e3e3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingRelationshipDomain_0041e073-1476-4787-8996-e7a648a4b6d6" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_HedgingRelationshipDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_b4e7606c-3c54-4288-9b2a-72a0f4b7e3e3" xlink:to="loc_us-gaap_HedgingRelationshipDomain_0041e073-1476-4787-8996-e7a648a4b6d6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashFlowHedgingMember_75d6acd6-154e-4112-a243-a5248dec5903" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CashFlowHedgingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingRelationshipDomain_0041e073-1476-4787-8996-e7a648a4b6d6" xlink:to="loc_us-gaap_CashFlowHedgingMember_75d6acd6-154e-4112-a243-a5248dec5903" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeNotionalAmount_d36948cd-e4d4-4862-ab53-24f829a124af" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DerivativeNotionalAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_3671778a-0ae4-4cac-a2a3-203d8b992ab9" xlink:to="loc_us-gaap_DerivativeNotionalAmount_d36948cd-e4d4-4862-ab53-24f829a124af" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeFloorInterestRate_71e17200-0163-4627-b970-9ea1daba45d0" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DerivativeFloorInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_3671778a-0ae4-4cac-a2a3-203d8b992ab9" xlink:to="loc_us-gaap_DerivativeFloorInterestRate_71e17200-0163-4627-b970-9ea1daba45d0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeAverageFixedInterestRate_346f83ce-5dac-49f5-a113-2271fe7ec7ab" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DerivativeAverageFixedInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_3671778a-0ae4-4cac-a2a3-203d8b992ab9" xlink:to="loc_us-gaap_DerivativeAverageFixedInterestRate_346f83ce-5dac-49f5-a113-2271fe7ec7ab" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/DEBTSummaryofCashFlowHedgesDetails" xlink:type="simple" xlink:href="cwst-20250331.xsd#DEBTSummaryofCashFlowHedgesDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/DEBTSummaryofCashFlowHedgesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativesFairValueLineItems_970d77ce-023d-49fb-a4e2-2fab4623b3f8" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DerivativesFairValueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_65b27e55-01ae-4c66-82c8-d8320ea221ba" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_970d77ce-023d-49fb-a4e2-2fab4623b3f8" xlink:to="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_65b27e55-01ae-4c66-82c8-d8320ea221ba" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationAxis_b335e1b4-353a-4dc5-a93b-f625ea06bf5c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_BalanceSheetLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_65b27e55-01ae-4c66-82c8-d8320ea221ba" xlink:to="loc_us-gaap_BalanceSheetLocationAxis_b335e1b4-353a-4dc5-a93b-f625ea06bf5c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_b335e1b4-353a-4dc5-a93b-f625ea06bf5c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_BalanceSheetLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_b335e1b4-353a-4dc5-a93b-f625ea06bf5c" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_b335e1b4-353a-4dc5-a93b-f625ea06bf5c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_35e1f30e-0b9b-44f3-b236-8f91ea495337" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_BalanceSheetLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_b335e1b4-353a-4dc5-a93b-f625ea06bf5c" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_35e1f30e-0b9b-44f3-b236-8f91ea495337" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCurrentAssetsMember_29a34a39-24fd-4fdf-97cd-a4033ffb8578" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherCurrentAssetsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_35e1f30e-0b9b-44f3-b236-8f91ea495337" xlink:to="loc_us-gaap_OtherCurrentAssetsMember_29a34a39-24fd-4fdf-97cd-a4033ffb8578" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNoncurrentAssetsMember_5c36e43b-2744-49c8-927d-95f3f54a7ee2" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherNoncurrentAssetsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_35e1f30e-0b9b-44f3-b236-8f91ea495337" xlink:to="loc_us-gaap_OtherNoncurrentAssetsMember_5c36e43b-2744-49c8-927d-95f3f54a7ee2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_OtherAccruedLiabilitiesMember_d378c03d-386c-4e8b-ba62-3d92ee3cb491" xlink:href="cwst-20250331.xsd#cwst_OtherAccruedLiabilitiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_35e1f30e-0b9b-44f3-b236-8f91ea495337" xlink:to="loc_cwst_OtherAccruedLiabilitiesMember_d378c03d-386c-4e8b-ba62-3d92ee3cb491" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNoncurrentLiabilitiesMember_b0603d6a-2d00-4cbf-b400-29f407d548fe" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherNoncurrentLiabilitiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_35e1f30e-0b9b-44f3-b236-8f91ea495337" xlink:to="loc_us-gaap_OtherNoncurrentLiabilitiesMember_b0603d6a-2d00-4cbf-b400-29f407d548fe" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationAxis_5c7fa387-60e3-461e-8b09-a90d7c5f6f21" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_HedgingDesignationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_65b27e55-01ae-4c66-82c8-d8320ea221ba" xlink:to="loc_us-gaap_HedgingDesignationAxis_5c7fa387-60e3-461e-8b09-a90d7c5f6f21" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationDomain_5c7fa387-60e3-461e-8b09-a90d7c5f6f21_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_HedgingDesignationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_HedgingDesignationAxis_5c7fa387-60e3-461e-8b09-a90d7c5f6f21" xlink:to="loc_us-gaap_HedgingDesignationDomain_5c7fa387-60e3-461e-8b09-a90d7c5f6f21_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationDomain_c6d95a32-b59a-4a03-90f4-576c9df855d3" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_HedgingDesignationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_HedgingDesignationAxis_5c7fa387-60e3-461e-8b09-a90d7c5f6f21" xlink:to="loc_us-gaap_HedgingDesignationDomain_c6d95a32-b59a-4a03-90f4-576c9df855d3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DesignatedAsHedgingInstrumentMember_229da5e1-ca3c-4006-8cf6-5ff6a831f6db" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DesignatedAsHedgingInstrumentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingDesignationDomain_c6d95a32-b59a-4a03-90f4-576c9df855d3" xlink:to="loc_us-gaap_DesignatedAsHedgingInstrumentMember_229da5e1-ca3c-4006-8cf6-5ff6a831f6db" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_861348de-72ac-4c85-8e49-d91e5e86ef66" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DerivativeInstrumentRiskAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_65b27e55-01ae-4c66-82c8-d8320ea221ba" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_861348de-72ac-4c85-8e49-d91e5e86ef66" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_861348de-72ac-4c85-8e49-d91e5e86ef66_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_861348de-72ac-4c85-8e49-d91e5e86ef66" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_861348de-72ac-4c85-8e49-d91e5e86ef66_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_144e1c54-590a-448e-a9d7-864b6a708e2f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_861348de-72ac-4c85-8e49-d91e5e86ef66" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_144e1c54-590a-448e-a9d7-864b6a708e2f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestRateSwapMember_7d6c88b3-94f0-4122-a462-71b9c75e9828" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_InterestRateSwapMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_144e1c54-590a-448e-a9d7-864b6a708e2f" xlink:to="loc_us-gaap_InterestRateSwapMember_7d6c88b3-94f0-4122-a462-71b9c75e9828" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeLiabilityFairValueGrossAsset_a775b4a7-8757-4ee5-8a7e-93b0cab7ce0e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DerivativeLiabilityFairValueGrossAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_970d77ce-023d-49fb-a4e2-2fab4623b3f8" xlink:to="loc_us-gaap_DerivativeLiabilityFairValueGrossAsset_a775b4a7-8757-4ee5-8a7e-93b0cab7ce0e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeFairValueOfDerivativeLiability_458f7f81-11c0-4192-b5ec-3d0aa1973dfd" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DerivativeFairValueOfDerivativeLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_970d77ce-023d-49fb-a4e2-2fab4623b3f8" xlink:to="loc_us-gaap_DerivativeFairValueOfDerivativeLiability_458f7f81-11c0-4192-b5ec-3d0aa1973dfd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AociBeforeTaxAttributableToParent_e4557575-3c12-434b-8c2a-776d16771ca0" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AociBeforeTaxAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_970d77ce-023d-49fb-a4e2-2fab4623b3f8" xlink:to="loc_us-gaap_AociBeforeTaxAttributableToParent_e4557575-3c12-434b-8c2a-776d16771ca0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AociTaxAttributableToParent_458a2736-e3d9-497e-be62-05646cc0a572" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AociTaxAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_970d77ce-023d-49fb-a4e2-2fab4623b3f8" xlink:to="loc_us-gaap_AociTaxAttributableToParent_458a2736-e3d9-497e-be62-05646cc0a572" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_e58a063b-a998-45e0-8772-6afc6a6d2d91" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_970d77ce-023d-49fb-a4e2-2fab4623b3f8" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_e58a063b-a998-45e0-8772-6afc6a6d2d91" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/COMMITMENTSANDCONTINGENCIESNarrativeDetails" xlink:type="simple" xlink:href="cwst-20250331.xsd#COMMITMENTSANDCONTINGENCIESNarrativeDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/COMMITMENTSANDCONTINGENCIESNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesLineItems_793b9e9f-091e-4338-98fb-68a28d87f06a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LossContingenciesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesTable_e80158c2-d6ef-49ef-bd6d-542944a84773" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LossContingenciesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LossContingenciesLineItems_793b9e9f-091e-4338-98fb-68a28d87f06a" xlink:to="loc_us-gaap_LossContingenciesTable_e80158c2-d6ef-49ef-bd6d-542944a84773" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_c7b025ee-63c0-4719-b1bf-7ef7eaa76416" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_e80158c2-d6ef-49ef-bd6d-542944a84773" xlink:to="loc_srt_RangeAxis_c7b025ee-63c0-4719-b1bf-7ef7eaa76416" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_c7b025ee-63c0-4719-b1bf-7ef7eaa76416_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_c7b025ee-63c0-4719-b1bf-7ef7eaa76416" xlink:to="loc_srt_RangeMember_c7b025ee-63c0-4719-b1bf-7ef7eaa76416_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_4db87710-9108-48bf-81d7-bff4e5cf5f12" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_c7b025ee-63c0-4719-b1bf-7ef7eaa76416" xlink:to="loc_srt_RangeMember_4db87710-9108-48bf-81d7-bff4e5cf5f12" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_53e502c0-a380-4d76-9cdd-a8e9269f6b4d" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_4db87710-9108-48bf-81d7-bff4e5cf5f12" xlink:to="loc_srt_MaximumMember_53e502c0-a380-4d76-9cdd-a8e9269f6b4d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_8f1bb916-2dd3-4743-b08c-50634d61aaab" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_4db87710-9108-48bf-81d7-bff4e5cf5f12" xlink:to="loc_srt_MinimumMember_8f1bb916-2dd3-4743-b08c-50634d61aaab" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_LitigationCaseAxis_ec5a9c5d-cc33-4f9a-aa5c-43e35f90b05c" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_LitigationCaseAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_e80158c2-d6ef-49ef-bd6d-542944a84773" xlink:to="loc_srt_LitigationCaseAxis_ec5a9c5d-cc33-4f9a-aa5c-43e35f90b05c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_LitigationCaseTypeDomain_ec5a9c5d-cc33-4f9a-aa5c-43e35f90b05c_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_LitigationCaseTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_LitigationCaseAxis_ec5a9c5d-cc33-4f9a-aa5c-43e35f90b05c" xlink:to="loc_srt_LitigationCaseTypeDomain_ec5a9c5d-cc33-4f9a-aa5c-43e35f90b05c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_LitigationCaseTypeDomain_b838923c-97e0-4160-a86e-945e10092be3" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_LitigationCaseTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_LitigationCaseAxis_ec5a9c5d-cc33-4f9a-aa5c-43e35f90b05c" xlink:to="loc_srt_LitigationCaseTypeDomain_b838923c-97e0-4160-a86e-945e10092be3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_NewHampshireDepartmentOfEnvironmentalServicesLetterOfDeficiencyMember_48d5e3a8-6e61-43c5-bb27-c1cb91819ac4" xlink:href="cwst-20250331.xsd#cwst_NewHampshireDepartmentOfEnvironmentalServicesLetterOfDeficiencyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_LitigationCaseTypeDomain_b838923c-97e0-4160-a86e-945e10092be3" xlink:to="loc_cwst_NewHampshireDepartmentOfEnvironmentalServicesLetterOfDeficiencyMember_48d5e3a8-6e61-43c5-bb27-c1cb91819ac4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingencyAccrualAtCarryingValue_1093939f-f673-4aa0-847b-22ad012624c3" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LossContingencyAccrualAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_793b9e9f-091e-4338-98fb-68a28d87f06a" xlink:to="loc_us-gaap_LossContingencyAccrualAtCarryingValue_1093939f-f673-4aa0-847b-22ad012624c3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_LossContingencyAccrualThreshold_b8db3e87-9d64-44ec-a554-6ea78c72b305" xlink:href="cwst-20250331.xsd#cwst_LossContingencyAccrualThreshold"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_793b9e9f-091e-4338-98fb-68a28d87f06a" xlink:to="loc_cwst_LossContingencyAccrualThreshold_b8db3e87-9d64-44ec-a554-6ea78c72b305" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_CapitalizedProjectDevelopmentCosts_b7fc5df5-70ca-41f4-92fa-ef5735eecf30" xlink:href="cwst-20250331.xsd#cwst_CapitalizedProjectDevelopmentCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_793b9e9f-091e-4338-98fb-68a28d87f06a" xlink:to="loc_cwst_CapitalizedProjectDevelopmentCosts_b7fc5df5-70ca-41f4-92fa-ef5735eecf30" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccrualForEnvironmentalLossContingenciesDiscountRate_51427d01-78f7-441d-9443-4e5ab5ebffd4" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AccrualForEnvironmentalLossContingenciesDiscountRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_793b9e9f-091e-4338-98fb-68a28d87f06a" xlink:to="loc_us-gaap_AccrualForEnvironmentalLossContingenciesDiscountRate_51427d01-78f7-441d-9443-4e5ab5ebffd4" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/STOCKHOLDERSEQUITYNarrativeDetail" xlink:type="simple" xlink:href="cwst-20250331.xsd#STOCKHOLDERSEQUITYNarrativeDetail"/>
  <link:definitionLink xlink:role="http://casella.com/role/STOCKHOLDERSEQUITYNarrativeDetail" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_d3bbbd44-929c-4bf5-97a5-5b2b462fe98e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LimitedPartnersCapitalAccountLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfLimitedPartnersCapitalAccountByClassTable_fd88358d-623c-4536-8109-6179e81607e9" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfLimitedPartnersCapitalAccountByClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_d3bbbd44-929c-4bf5-97a5-5b2b462fe98e" xlink:to="loc_us-gaap_ScheduleOfLimitedPartnersCapitalAccountByClassTable_fd88358d-623c-4536-8109-6179e81607e9" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_d9ca37d1-eb74-44bc-9d97-fe0ba5f4c3e5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfLimitedPartnersCapitalAccountByClassTable_fd88358d-623c-4536-8109-6179e81607e9" xlink:to="loc_us-gaap_StatementClassOfStockAxis_d9ca37d1-eb74-44bc-9d97-fe0ba5f4c3e5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_d9ca37d1-eb74-44bc-9d97-fe0ba5f4c3e5_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_d9ca37d1-eb74-44bc-9d97-fe0ba5f4c3e5" xlink:to="loc_us-gaap_ClassOfStockDomain_d9ca37d1-eb74-44bc-9d97-fe0ba5f4c3e5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_f6b23438-1454-4580-93e5-004805b5a794" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_d9ca37d1-eb74-44bc-9d97-fe0ba5f4c3e5" xlink:to="loc_us-gaap_ClassOfStockDomain_f6b23438-1454-4580-93e5-004805b5a794" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_dd099d2f-0cfc-479c-bff7-c20a9ea9d1fe" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_f6b23438-1454-4580-93e5-004805b5a794" xlink:to="loc_us-gaap_CommonClassAMember_dd099d2f-0cfc-479c-bff7-c20a9ea9d1fe" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_97f7f2ac-516c-4196-a2c5-013f2b1c4f22" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfLimitedPartnersCapitalAccountByClassTable_fd88358d-623c-4536-8109-6179e81607e9" xlink:to="loc_us-gaap_PlanNameAxis_97f7f2ac-516c-4196-a2c5-013f2b1c4f22" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_97f7f2ac-516c-4196-a2c5-013f2b1c4f22_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_97f7f2ac-516c-4196-a2c5-013f2b1c4f22" xlink:to="loc_us-gaap_PlanNameDomain_97f7f2ac-516c-4196-a2c5-013f2b1c4f22_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_2c5fc966-69a1-4456-a336-cc9d38cc6446" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_97f7f2ac-516c-4196-a2c5-013f2b1c4f22" xlink:to="loc_us-gaap_PlanNameDomain_2c5fc966-69a1-4456-a336-cc9d38cc6446" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_Amended2016PlanMember_bd2a0559-1d92-47d4-a684-20f8d958c1ac" xlink:href="cwst-20250331.xsd#cwst_Amended2016PlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_2c5fc966-69a1-4456-a336-cc9d38cc6446" xlink:to="loc_cwst_Amended2016PlanMember_bd2a0559-1d92-47d4-a684-20f8d958c1ac" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_A2016IncentivePlanMember_55f46f1b-d11b-422b-a79f-a0f4f715b049" xlink:href="cwst-20250331.xsd#cwst_A2016IncentivePlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_2c5fc966-69a1-4456-a336-cc9d38cc6446" xlink:to="loc_cwst_A2016IncentivePlanMember_55f46f1b-d11b-422b-a79f-a0f4f715b049" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_A2006IncentivePlanMember_a9ab33f0-95c4-4f29-896c-4fd7bd4a81aa" xlink:href="cwst-20250331.xsd#cwst_A2006IncentivePlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_2c5fc966-69a1-4456-a336-cc9d38cc6446" xlink:to="loc_cwst_A2006IncentivePlanMember_a9ab33f0-95c4-4f29-896c-4fd7bd4a81aa" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_AmendedAndRestatedNineteenNinetySevenEmployeeStockPurchasePlanMember_6982746c-a77a-4d21-b333-8e4be59a86f1" xlink:href="cwst-20250331.xsd#cwst_AmendedAndRestatedNineteenNinetySevenEmployeeStockPurchasePlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_2c5fc966-69a1-4456-a336-cc9d38cc6446" xlink:to="loc_cwst_AmendedAndRestatedNineteenNinetySevenEmployeeStockPurchasePlanMember_6982746c-a77a-4d21-b333-8e4be59a86f1" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_ba525c65-e2d1-4871-8383-d5310d1edeab" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfLimitedPartnersCapitalAccountByClassTable_fd88358d-623c-4536-8109-6179e81607e9" xlink:to="loc_srt_RangeAxis_ba525c65-e2d1-4871-8383-d5310d1edeab" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_ba525c65-e2d1-4871-8383-d5310d1edeab_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_ba525c65-e2d1-4871-8383-d5310d1edeab" xlink:to="loc_srt_RangeMember_ba525c65-e2d1-4871-8383-d5310d1edeab_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_36ecf222-c4e4-407e-a3be-57e87524d12c" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_ba525c65-e2d1-4871-8383-d5310d1edeab" xlink:to="loc_srt_RangeMember_36ecf222-c4e4-407e-a3be-57e87524d12c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_770cf252-4597-4eea-9207-3a534d970d39" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_36ecf222-c4e4-407e-a3be-57e87524d12c" xlink:to="loc_srt_MinimumMember_770cf252-4597-4eea-9207-3a534d970d39" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_61a5d8ac-5723-43b0-906a-caf431553c3a" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_36ecf222-c4e4-407e-a3be-57e87524d12c" xlink:to="loc_srt_MaximumMember_61a5d8ac-5723-43b0-906a-caf431553c3a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_WeightedAverageMember_759b59db-7793-40e4-ac9a-905be222076c" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_WeightedAverageMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_36ecf222-c4e4-407e-a3be-57e87524d12c" xlink:to="loc_srt_WeightedAverageMember_759b59db-7793-40e4-ac9a-905be222076c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_a78f616b-0ff0-409f-8cb9-3fc63748645c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfLimitedPartnersCapitalAccountByClassTable_fd88358d-623c-4536-8109-6179e81607e9" xlink:to="loc_us-gaap_AwardTypeAxis_a78f616b-0ff0-409f-8cb9-3fc63748645c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_a78f616b-0ff0-409f-8cb9-3fc63748645c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_a78f616b-0ff0-409f-8cb9-3fc63748645c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_a78f616b-0ff0-409f-8cb9-3fc63748645c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_836e6bba-5299-489c-b5d9-bcdc7c809f1b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_a78f616b-0ff0-409f-8cb9-3fc63748645c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_836e6bba-5299-489c-b5d9-bcdc7c809f1b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_08cc2a7d-c9af-46dc-b2ef-2334cdfb3783" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_836e6bba-5299-489c-b5d9-bcdc7c809f1b" xlink:to="loc_us-gaap_EmployeeStockOptionMember_08cc2a7d-c9af-46dc-b2ef-2334cdfb3783" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_RestrictedStockAwardsMember_93e3fb1b-f970-42c5-81d2-bb57a351f32c" xlink:href="cwst-20250331.xsd#cwst_RestrictedStockAwardsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_836e6bba-5299-489c-b5d9-bcdc7c809f1b" xlink:to="loc_cwst_RestrictedStockAwardsMember_93e3fb1b-f970-42c5-81d2-bb57a351f32c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_4a908bb0-4c95-408f-9a63-1b6fc0aef6db" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_836e6bba-5299-489c-b5d9-bcdc7c809f1b" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_4a908bb0-4c95-408f-9a63-1b6fc0aef6db" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_RestrictedStockRestrictedStockUnitsAndPerformanceStockUnitsMember_5c57774b-b99c-4ac4-98b1-eb3b5682e30d" xlink:href="cwst-20250331.xsd#cwst_RestrictedStockRestrictedStockUnitsAndPerformanceStockUnitsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_836e6bba-5299-489c-b5d9-bcdc7c809f1b" xlink:to="loc_cwst_RestrictedStockRestrictedStockUnitsAndPerformanceStockUnitsMember_5c57774b-b99c-4ac4-98b1-eb3b5682e30d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_PerformanceStockUnitMember_485bb146-ca89-4f2d-be72-e47e55a74d45" xlink:href="cwst-20250331.xsd#cwst_PerformanceStockUnitMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_836e6bba-5299-489c-b5d9-bcdc7c809f1b" xlink:to="loc_cwst_PerformanceStockUnitMember_485bb146-ca89-4f2d-be72-e47e55a74d45" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_MarketbasedPerformanceStockUnitsMember_15fae88f-1dbc-4366-9771-6f43e0729199" xlink:href="cwst-20250331.xsd#cwst_MarketbasedPerformanceStockUnitsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_836e6bba-5299-489c-b5d9-bcdc7c809f1b" xlink:to="loc_cwst_MarketbasedPerformanceStockUnitsMember_15fae88f-1dbc-4366-9771-6f43e0729199" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualAxis_f69ca840-d64c-4d69-be38-a1615ce9d13d" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_TitleOfIndividualAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfLimitedPartnersCapitalAccountByClassTable_fd88358d-623c-4536-8109-6179e81607e9" xlink:to="loc_srt_TitleOfIndividualAxis_f69ca840-d64c-4d69-be38-a1615ce9d13d" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_f69ca840-d64c-4d69-be38-a1615ce9d13d_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_TitleOfIndividualAxis_f69ca840-d64c-4d69-be38-a1615ce9d13d" xlink:to="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_f69ca840-d64c-4d69-be38-a1615ce9d13d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_10112ee9-e320-422f-bbab-51e11c566fbc" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_TitleOfIndividualAxis_f69ca840-d64c-4d69-be38-a1615ce9d13d" xlink:to="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_10112ee9-e320-422f-bbab-51e11c566fbc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_NonEmployeeDirectorMember_d20b4f8d-8a90-4457-8ba9-1507c11e43e6" xlink:href="cwst-20250331.xsd#cwst_NonEmployeeDirectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_10112ee9-e320-422f-bbab-51e11c566fbc" xlink:to="loc_cwst_NonEmployeeDirectorMember_d20b4f8d-8a90-4457-8ba9-1507c11e43e6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_5fc79186-81a6-4257-82d6-2101b342cfc7" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_d3bbbd44-929c-4bf5-97a5-5b2b462fe98e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_5fc79186-81a6-4257-82d6-2101b342cfc7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized_b3e4d6d6-3bf3-4628-aa2f-76b57bcd65db" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_d3bbbd44-929c-4bf5-97a5-5b2b462fe98e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized_b3e4d6d6-3bf3-4628-aa2f-76b57bcd65db" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_4c327f56-94d2-49cb-b3d8-2c0c4faff44f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_d3bbbd44-929c-4bf5-97a5-5b2b462fe98e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_4c327f56-94d2-49cb-b3d8-2c0c4faff44f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_ac4a8222-80bf-4827-ac9c-770aacd01bd0" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_d3bbbd44-929c-4bf5-97a5-5b2b462fe98e" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_ac4a8222-80bf-4827-ac9c-770aacd01bd0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_e7a7e50a-d3a8-4d03-b675-21507c0c538a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_d3bbbd44-929c-4bf5-97a5-5b2b462fe98e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_e7a7e50a-d3a8-4d03-b675-21507c0c538a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_fcc13902-41c6-4dd4-886f-f85f67e15de9" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_d3bbbd44-929c-4bf5-97a5-5b2b462fe98e" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_fcc13902-41c6-4dd4-886f-f85f67e15de9" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_1cfa987a-da5f-4b27-b4f5-494d38e6b059" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_d3bbbd44-929c-4bf5-97a5-5b2b462fe98e" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_1cfa987a-da5f-4b27-b4f5-494d38e6b059" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_99732feb-d058-454c-aba6-9fabac74c994" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_d3bbbd44-929c-4bf5-97a5-5b2b462fe98e" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_99732feb-d058-454c-aba6-9fabac74c994" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue_ed76893d-2157-406c-840e-93236c093aa5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_d3bbbd44-929c-4bf5-97a5-5b2b462fe98e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue_ed76893d-2157-406c-840e-93236c093aa5" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardRequisiteServicePeriod1_9c0832da-f179-460e-a31e-9fb5b9bafbf5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardRequisiteServicePeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_d3bbbd44-929c-4bf5-97a5-5b2b462fe98e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardRequisiteServicePeriod1_9c0832da-f179-460e-a31e-9fb5b9bafbf5" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue_f4ec8634-8c18-41ac-862d-a847232b1729" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_d3bbbd44-929c-4bf5-97a5-5b2b462fe98e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue_f4ec8634-8c18-41ac-862d-a847232b1729" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_cb426b93-34a2-49c3-a339-fd86a1d6b579" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_d3bbbd44-929c-4bf5-97a5-5b2b462fe98e" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_cb426b93-34a2-49c3-a339-fd86a1d6b579" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_b0ac2ae5-84f1-4a39-98af-42b8375cbfbd" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_d3bbbd44-929c-4bf5-97a5-5b2b462fe98e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_b0ac2ae5-84f1-4a39-98af-42b8375cbfbd" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_218de352-9058-41d5-bb67-3685571f8b4e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_d3bbbd44-929c-4bf5-97a5-5b2b462fe98e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_218de352-9058-41d5-bb67-3685571f8b4e" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_b0e1471e-4d76-4cac-b9d2-32f74120d2e7" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_d3bbbd44-929c-4bf5-97a5-5b2b462fe98e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_b0e1471e-4d76-4cac-b9d2-32f74120d2e7" xlink:type="arc" order="14"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/STOCKHOLDERSEQUITYSummaryofRestrictedStockAwardsRestrictedStockUnitandPerformancebasedStockUnitActivityDetail" xlink:type="simple" xlink:href="cwst-20250331.xsd#STOCKHOLDERSEQUITYSummaryofRestrictedStockAwardsRestrictedStockUnitandPerformancebasedStockUnitActivityDetail"/>
  <link:definitionLink xlink:role="http://casella.com/role/STOCKHOLDERSEQUITYSummaryofRestrictedStockAwardsRestrictedStockUnitandPerformancebasedStockUnitActivityDetail" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_05859304-ac26-4e77-af35-bc4476a4b73c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_8090d4d4-5224-4948-85ba-6a1b8a05718f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_05859304-ac26-4e77-af35-bc4476a4b73c" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_8090d4d4-5224-4948-85ba-6a1b8a05718f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingAxis_8bb48c83-9cdc-4ae7-a8dd-8dc68b648621" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_VestingAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_8090d4d4-5224-4948-85ba-6a1b8a05718f" xlink:to="loc_us-gaap_VestingAxis_8bb48c83-9cdc-4ae7-a8dd-8dc68b648621" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingDomain_8bb48c83-9cdc-4ae7-a8dd-8dc68b648621_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_VestingDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VestingAxis_8bb48c83-9cdc-4ae7-a8dd-8dc68b648621" xlink:to="loc_us-gaap_VestingDomain_8bb48c83-9cdc-4ae7-a8dd-8dc68b648621_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingDomain_f9867814-b9f7-43f5-8f23-b1eabce6bb77" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_VestingDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VestingAxis_8bb48c83-9cdc-4ae7-a8dd-8dc68b648621" xlink:to="loc_us-gaap_VestingDomain_f9867814-b9f7-43f5-8f23-b1eabce6bb77" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationAwardTrancheOneMember_159a6a66-b058-4a06-83d6-7e1b3d41bc0f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationAwardTrancheOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_f9867814-b9f7-43f5-8f23-b1eabce6bb77" xlink:to="loc_us-gaap_ShareBasedCompensationAwardTrancheOneMember_159a6a66-b058-4a06-83d6-7e1b3d41bc0f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_9d8e52e2-9551-4085-b96d-05697ba18a4d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_8090d4d4-5224-4948-85ba-6a1b8a05718f" xlink:to="loc_us-gaap_AwardTypeAxis_9d8e52e2-9551-4085-b96d-05697ba18a4d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_9d8e52e2-9551-4085-b96d-05697ba18a4d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_9d8e52e2-9551-4085-b96d-05697ba18a4d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_9d8e52e2-9551-4085-b96d-05697ba18a4d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_f65c253a-8ec2-4cf9-84c9-a9dbd104b841" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_9d8e52e2-9551-4085-b96d-05697ba18a4d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_f65c253a-8ec2-4cf9-84c9-a9dbd104b841" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_PerformanceStockUnitMember_29e8cb80-c1a1-4f47-bf66-e991c3ce642f" xlink:href="cwst-20250331.xsd#cwst_PerformanceStockUnitMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_f65c253a-8ec2-4cf9-84c9-a9dbd104b841" xlink:to="loc_cwst_PerformanceStockUnitMember_29e8cb80-c1a1-4f47-bf66-e991c3ce642f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_RestrictedStockRestrictedStockUnitsAndPerformanceStockUnitsMember_a5e6033b-e926-4f52-b6ba-26fda47c782d" xlink:href="cwst-20250331.xsd#cwst_RestrictedStockRestrictedStockUnitsAndPerformanceStockUnitsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cwst_PerformanceStockUnitMember_29e8cb80-c1a1-4f47-bf66-e991c3ce642f" xlink:to="loc_cwst_RestrictedStockRestrictedStockUnitsAndPerformanceStockUnitsMember_a5e6033b-e926-4f52-b6ba-26fda47c782d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_296497dd-d7b1-413c-a6f9-32cc00be8f88" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_8090d4d4-5224-4948-85ba-6a1b8a05718f" xlink:to="loc_us-gaap_StatementClassOfStockAxis_296497dd-d7b1-413c-a6f9-32cc00be8f88" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_296497dd-d7b1-413c-a6f9-32cc00be8f88_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_296497dd-d7b1-413c-a6f9-32cc00be8f88" xlink:to="loc_us-gaap_ClassOfStockDomain_296497dd-d7b1-413c-a6f9-32cc00be8f88_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_011c250c-e0e2-4c94-849a-f6ccc6b0e4f6" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_296497dd-d7b1-413c-a6f9-32cc00be8f88" xlink:to="loc_us-gaap_ClassOfStockDomain_011c250c-e0e2-4c94-849a-f6ccc6b0e4f6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_98c289b4-d6b4-40ce-be60-54429cce719d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_011c250c-e0e2-4c94-849a-f6ccc6b0e4f6" xlink:to="loc_us-gaap_CommonClassAMember_98c289b4-d6b4-40ce-be60-54429cce719d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_d96bdb3c-6b9b-406a-a253-33b0d25bb501" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_05859304-ac26-4e77-af35-bc4476a4b73c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_d96bdb3c-6b9b-406a-a253-33b0d25bb501" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_5e3b50dd-929c-4e70-aee0-52692dd68ddd" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_d96bdb3c-6b9b-406a-a253-33b0d25bb501" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_5e3b50dd-929c-4e70-aee0-52692dd68ddd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_1a6cc3bf-09e9-4832-ba93-5216a4452174" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_d96bdb3c-6b9b-406a-a253-33b0d25bb501" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_1a6cc3bf-09e9-4832-ba93-5216a4452174" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_f1294088-aa27-4c1e-9643-08d03336d8ee" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_d96bdb3c-6b9b-406a-a253-33b0d25bb501" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_f1294088-aa27-4c1e-9643-08d03336d8ee" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_28e9c5e6-9b95-435b-b67b-c13e1fa70039" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_d96bdb3c-6b9b-406a-a253-33b0d25bb501" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_28e9c5e6-9b95-435b-b67b-c13e1fa70039" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_42c365b5-66f4-4b82-9ef1-1ba9cf7e7c27" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_a5685536-d99e-453d-b73c-06b600b90c1d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_05859304-ac26-4e77-af35-bc4476a4b73c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_a5685536-d99e-453d-b73c-06b600b90c1d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_0a9766a3-61c4-429b-b835-602cdc660af1" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_a5685536-d99e-453d-b73c-06b600b90c1d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_0a9766a3-61c4-429b-b835-602cdc660af1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_73ab33b2-094d-4f87-ab23-a4c5cb1474f8" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_a5685536-d99e-453d-b73c-06b600b90c1d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_73ab33b2-094d-4f87-ab23-a4c5cb1474f8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_6a7c8b9a-5acd-41ef-bb7f-b72a922b3d41" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_a5685536-d99e-453d-b73c-06b600b90c1d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_6a7c8b9a-5acd-41ef-bb7f-b72a922b3d41" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_ab51d422-a543-4a01-be69-7800cc5523c8" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_a5685536-d99e-453d-b73c-06b600b90c1d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_ab51d422-a543-4a01-be69-7800cc5523c8" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_e940cec5-e07a-4565-8ba3-a63ad15a651e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalDisclosuresAbstract_8d899a0d-a73b-4dec-a0d8-78dcc44e8272" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalDisclosuresAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_05859304-ac26-4e77-af35-bc4476a4b73c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalDisclosuresAbstract_8d899a0d-a73b-4dec-a0d8-78dcc44e8272" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingWeightedAverageRemainingContractualTerms_427912c4-f315-4ce9-b829-8b8a5037d9c3" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingWeightedAverageRemainingContractualTerms"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalDisclosuresAbstract_8d899a0d-a73b-4dec-a0d8-78dcc44e8272" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingWeightedAverageRemainingContractualTerms_427912c4-f315-4ce9-b829-8b8a5037d9c3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueOutstanding_bd24fe99-cc4d-409a-a52d-ef3b706eb608" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalDisclosuresAbstract_8d899a0d-a73b-4dec-a0d8-78dcc44e8272" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueOutstanding_bd24fe99-cc4d-409a-a52d-ef3b706eb608" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_a14ccfd4-0142-4c32-8366-2ce72047516e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_05859304-ac26-4e77-af35-bc4476a4b73c" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_a14ccfd4-0142-4c32-8366-2ce72047516e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_AdditionalSharesIssuedMaximumAttainmentLevel_5ea11669-b601-4822-ba73-797b830782e7" xlink:href="cwst-20250331.xsd#cwst_AdditionalSharesIssuedMaximumAttainmentLevel"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_05859304-ac26-4e77-af35-bc4476a4b73c" xlink:to="loc_cwst_AdditionalSharesIssuedMaximumAttainmentLevel_5ea11669-b601-4822-ba73-797b830782e7" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/STOCKHOLDERSEQUITYSummaryofChangesinBalancesofEachComponentofAccumulatedOtherComprehensiveIncomeLossDetails" xlink:type="simple" xlink:href="cwst-20250331.xsd#STOCKHOLDERSEQUITYSummaryofChangesinBalancesofEachComponentofAccumulatedOtherComprehensiveIncomeLossDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/STOCKHOLDERSEQUITYSummaryofChangesinBalancesofEachComponentofAccumulatedOtherComprehensiveIncomeLossDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_b07fcd44-3012-4c34-bc33-2d978e32a4e1" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_73a7b6a3-fac3-4cb0-8009-68bbbf2d00a4" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_b07fcd44-3012-4c34-bc33-2d978e32a4e1" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_73a7b6a3-fac3-4cb0-8009-68bbbf2d00a4" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_56562c14-ee79-496d-9844-ca26babc0d5d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_73a7b6a3-fac3-4cb0-8009-68bbbf2d00a4" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_56562c14-ee79-496d-9844-ca26babc0d5d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_56562c14-ee79-496d-9844-ca26babc0d5d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_56562c14-ee79-496d-9844-ca26babc0d5d" xlink:to="loc_us-gaap_EquityComponentDomain_56562c14-ee79-496d-9844-ca26babc0d5d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_7c5bdaad-38a1-4034-925e-a4ff3d2903b5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_56562c14-ee79-496d-9844-ca26babc0d5d" xlink:to="loc_us-gaap_EquityComponentDomain_7c5bdaad-38a1-4034-925e-a4ff3d2903b5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedGainLossNetCashFlowHedgeParentMember_654af771-bcb6-4ccb-9e57-8c8263bd8e59" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AccumulatedGainLossNetCashFlowHedgeParentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_7c5bdaad-38a1-4034-925e-a4ff3d2903b5" xlink:to="loc_us-gaap_AccumulatedGainLossNetCashFlowHedgeParentMember_654af771-bcb6-4ccb-9e57-8c8263bd8e59" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTaxAbstract_11a88dca-7493-4cfa-9028-b11bd6c0b53c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTaxAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_b07fcd44-3012-4c34-bc33-2d978e32a4e1" xlink:to="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTaxAbstract_11a88dca-7493-4cfa-9028-b11bd6c0b53c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_461883d2-cb9f-4612-9dfc-4c26716f9252" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTaxAbstract_11a88dca-7493-4cfa-9028-b11bd6c0b53c" xlink:to="loc_us-gaap_StockholdersEquity_461883d2-cb9f-4612-9dfc-4c26716f9252" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OciBeforeReclassificationsBeforeTaxAttributableToParent_15199d70-2273-4c78-9dac-ebad9f89ac22" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OciBeforeReclassificationsBeforeTaxAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTaxAbstract_11a88dca-7493-4cfa-9028-b11bd6c0b53c" xlink:to="loc_us-gaap_OciBeforeReclassificationsBeforeTaxAttributableToParent_15199d70-2273-4c78-9dac-ebad9f89ac22" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationFromAociCurrentPeriodBeforeTaxAttributableToParent_a6a89650-115f-4588-a1e1-8a45cf3f2e8e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ReclassificationFromAociCurrentPeriodBeforeTaxAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTaxAbstract_11a88dca-7493-4cfa-9028-b11bd6c0b53c" xlink:to="loc_us-gaap_ReclassificationFromAociCurrentPeriodBeforeTaxAttributableToParent_a6a89650-115f-4588-a1e1-8a45cf3f2e8e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossTax_96b1820d-dfdf-4223-b61c-9a59bbcc29e5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherComprehensiveIncomeLossTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTaxAbstract_11a88dca-7493-4cfa-9028-b11bd6c0b53c" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossTax_96b1820d-dfdf-4223-b61c-9a59bbcc29e5" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_13017cfd-4785-4c99-90a1-cdfb30d0e6f5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTaxAbstract_11a88dca-7493-4cfa-9028-b11bd6c0b53c" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_13017cfd-4785-4c99-90a1-cdfb30d0e6f5" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_dc620f61-3e17-4d45-95d8-98eb67b81a54" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StockholdersEquity"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/STOCKHOLDERSEQUITYSummaryofReclassificationsOutofAccumulatedOtherComprehensiveLossDetails" xlink:type="simple" xlink:href="cwst-20250331.xsd#STOCKHOLDERSEQUITYSummaryofReclassificationsOutofAccumulatedOtherComprehensiveLossDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/STOCKHOLDERSEQUITYSummaryofReclassificationsOutofAccumulatedOtherComprehensiveLossDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_e1f9102a-9306-414e-a2c6-6e685dc67a2d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeTable_e375dac6-2199-43ed-af97-b6150b460481" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_e1f9102a-9306-414e-a2c6-6e685dc67a2d" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeTable_e375dac6-2199-43ed-af97-b6150b460481" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_0c902a3e-c790-4d7d-be81-646ac14cc31e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DerivativeInstrumentRiskAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeTable_e375dac6-2199-43ed-af97-b6150b460481" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_0c902a3e-c790-4d7d-be81-646ac14cc31e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_0c902a3e-c790-4d7d-be81-646ac14cc31e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_0c902a3e-c790-4d7d-be81-646ac14cc31e" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_0c902a3e-c790-4d7d-be81-646ac14cc31e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_1d3472bf-dbe2-48b9-891f-b66be68ac251" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_0c902a3e-c790-4d7d-be81-646ac14cc31e" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_1d3472bf-dbe2-48b9-891f-b66be68ac251" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestRateSwapMember_e387322a-14c5-47d6-8ea8-f43067b76847" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_InterestRateSwapMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_1d3472bf-dbe2-48b9-891f-b66be68ac251" xlink:to="loc_us-gaap_InterestRateSwapMember_e387322a-14c5-47d6-8ea8-f43067b76847" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_4de9c0e4-8d44-469a-bc85-26947ffa1417" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeTable_e375dac6-2199-43ed-af97-b6150b460481" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_4de9c0e4-8d44-469a-bc85-26947ffa1417" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_4de9c0e4-8d44-469a-bc85-26947ffa1417_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_4de9c0e4-8d44-469a-bc85-26947ffa1417" xlink:to="loc_us-gaap_EquityComponentDomain_4de9c0e4-8d44-469a-bc85-26947ffa1417_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_398cbcac-bc67-4295-80ce-6bf47ebdb0c0" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_4de9c0e4-8d44-469a-bc85-26947ffa1417" xlink:to="loc_us-gaap_EquityComponentDomain_398cbcac-bc67-4295-80ce-6bf47ebdb0c0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedGainLossNetCashFlowHedgeParentMember_67594ab5-9f50-4dcf-b018-ddb1006db345" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AccumulatedGainLossNetCashFlowHedgeParentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_398cbcac-bc67-4295-80ce-6bf47ebdb0c0" xlink:to="loc_us-gaap_AccumulatedGainLossNetCashFlowHedgeParentMember_67594ab5-9f50-4dcf-b018-ddb1006db345" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis_c643004f-7ef1-4c88-bab2-7e136dd4a7bd" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeTable_e375dac6-2199-43ed-af97-b6150b460481" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis_c643004f-7ef1-4c88-bab2-7e136dd4a7bd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_c643004f-7ef1-4c88-bab2-7e136dd4a7bd_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis_c643004f-7ef1-4c88-bab2-7e136dd4a7bd" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_c643004f-7ef1-4c88-bab2-7e136dd4a7bd_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_e68ff00d-4a80-4ffa-95cf-646e819f588f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis_c643004f-7ef1-4c88-bab2-7e136dd4a7bd" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_e68ff00d-4a80-4ffa-95cf-646e819f588f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember_61bf2e26-ad35-45b9-b813-027d4a06343b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_e68ff00d-4a80-4ffa-95cf-646e819f588f" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember_61bf2e26-ad35-45b9-b813-027d4a06343b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseNonoperating_f26d0691-b656-40c0-98ff-9125b4e916e6" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_InterestExpenseNonoperating"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_e1f9102a-9306-414e-a2c6-6e685dc67a2d" xlink:to="loc_us-gaap_InterestExpenseNonoperating_f26d0691-b656-40c0-98ff-9125b4e916e6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_0f3e6c00-0f89-4b87-939d-ff42aad331c6" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_e1f9102a-9306-414e-a2c6-6e685dc67a2d" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_0f3e6c00-0f89-4b87-939d-ff42aad331c6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_2a635e21-d27c-48dc-848c-df2dbf310818" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeTaxExpenseBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_e1f9102a-9306-414e-a2c6-6e685dc67a2d" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_2a635e21-d27c-48dc-848c-df2dbf310818" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_10b39b97-c166-4897-aad3-832b26ff6b57" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_e1f9102a-9306-414e-a2c6-6e685dc67a2d" xlink:to="loc_us-gaap_NetIncomeLoss_10b39b97-c166-4897-aad3-832b26ff6b57" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/LOSSPERSHAREDetails" xlink:type="simple" xlink:href="cwst-20250331.xsd#LOSSPERSHAREDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/LOSSPERSHAREDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_8353ee2c-de33-4058-8ff0-b00c5ccccc45" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_79d4065c-0873-4d56-b9dd-f7a908b8a6ef" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_8353ee2c-de33-4058-8ff0-b00c5ccccc45" xlink:to="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_79d4065c-0873-4d56-b9dd-f7a908b8a6ef" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_2939e196-7e94-4221-8f6e-386ddd6884fb" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_79d4065c-0873-4d56-b9dd-f7a908b8a6ef" xlink:to="loc_us-gaap_StatementClassOfStockAxis_2939e196-7e94-4221-8f6e-386ddd6884fb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_2939e196-7e94-4221-8f6e-386ddd6884fb_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_2939e196-7e94-4221-8f6e-386ddd6884fb" xlink:to="loc_us-gaap_ClassOfStockDomain_2939e196-7e94-4221-8f6e-386ddd6884fb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_d199c8dc-2187-43b3-9bbb-913bb61fb638" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_2939e196-7e94-4221-8f6e-386ddd6884fb" xlink:to="loc_us-gaap_ClassOfStockDomain_d199c8dc-2187-43b3-9bbb-913bb61fb638" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_23bdd01b-88fa-4416-84a9-13aabf97fcf1" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_d199c8dc-2187-43b3-9bbb-913bb61fb638" xlink:to="loc_us-gaap_CommonClassAMember_23bdd01b-88fa-4416-84a9-13aabf97fcf1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_24e6b65d-c7f5-4331-a6c7-926b8fe59315" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_d199c8dc-2187-43b3-9bbb-913bb61fb638" xlink:to="loc_us-gaap_CommonClassBMember_24e6b65d-c7f5-4331-a6c7-926b8fe59315" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersDilutedAbstract_6f302a10-fc27-40cd-a1a3-c94fd4a1ec3b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_NetIncomeLossAvailableToCommonStockholdersDilutedAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_8353ee2c-de33-4058-8ff0-b00c5ccccc45" xlink:to="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersDilutedAbstract_6f302a10-fc27-40cd-a1a3-c94fd4a1ec3b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperations_ebfbf35d-1572-40f4-892b-0976b3ba24eb" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeLossFromContinuingOperations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersDilutedAbstract_6f302a10-fc27-40cd-a1a3-c94fd4a1ec3b" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperations_ebfbf35d-1572-40f4-892b-0976b3ba24eb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract_7622661b-2a8f-416d-bce1-46ab3660cc35" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_8353ee2c-de33-4058-8ff0-b00c5ccccc45" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract_7622661b-2a8f-416d-bce1-46ab3660cc35" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_e6290186-1fd3-4e02-b94d-e577c46b7f3f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract_7622661b-2a8f-416d-bce1-46ab3660cc35" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_e6290186-1fd3-4e02-b94d-e577c46b7f3f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesIssuedBasic_b5a30c07-e9ee-4c0b-a405-745b8783ed68" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_WeightedAverageNumberOfSharesIssuedBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract_7622661b-2a8f-416d-bce1-46ab3660cc35" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesIssuedBasic_b5a30c07-e9ee-4c0b-a405-745b8783ed68" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_46fd596a-c559-484c-8a8f-50d560dc79b2" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_8353ee2c-de33-4058-8ff0-b00c5ccccc45" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_46fd596a-c559-484c-8a8f-50d560dc79b2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustmentAbstract_4001be20-cf3e-4a35-aa16-aeb8e7d4e684" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustmentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_8353ee2c-de33-4058-8ff0-b00c5ccccc45" xlink:to="loc_us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustmentAbstract_4001be20-cf3e-4a35-aa16-aeb8e7d4e684" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements_85d843af-edfa-49ff-ad16-23168666f342" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustmentAbstract_4001be20-cf3e-4a35-aa16-aeb8e7d4e684" xlink:to="loc_us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements_85d843af-edfa-49ff-ad16-23168666f342" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_8dfbb318-f2a6-4118-871e-9f0db303deca" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_8353ee2c-de33-4058-8ff0-b00c5ccccc45" xlink:to="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_8dfbb318-f2a6-4118-871e-9f0db303deca" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_277243e6-8ced-4662-8edc-554e9af4708f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_8353ee2c-de33-4058-8ff0-b00c5ccccc45" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_277243e6-8ced-4662-8edc-554e9af4708f" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/FAIRVALUEOFFINANCIALINSTRUMENTSRecurringFairValueMeasurementsDetail" xlink:type="simple" xlink:href="cwst-20250331.xsd#FAIRVALUEOFFINANCIALINSTRUMENTSRecurringFairValueMeasurementsDetail"/>
  <link:definitionLink xlink:role="http://casella.com/role/FAIRVALUEOFFINANCIALINSTRUMENTSRecurringFairValueMeasurementsDetail" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_bdb78d06-4c3a-4437-ba71-02a57b6a9e4b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_0197a1c2-ffa1-404c-aa76-e8fb84554ab5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_bdb78d06-4c3a-4437-ba71-02a57b6a9e4b" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_0197a1c2-ffa1-404c-aa76-e8fb84554ab5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementFrequencyAxis_65fdfc80-f44b-4278-9403-7b42597febdd" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueByMeasurementFrequencyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_0197a1c2-ffa1-404c-aa76-e8fb84554ab5" xlink:to="loc_us-gaap_FairValueByMeasurementFrequencyAxis_65fdfc80-f44b-4278-9403-7b42597febdd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_65fdfc80-f44b-4278-9403-7b42597febdd_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_65fdfc80-f44b-4278-9403-7b42597febdd" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_65fdfc80-f44b-4278-9403-7b42597febdd_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_1aab9999-85bb-4a3c-a26d-c153b734bc4e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_65fdfc80-f44b-4278-9403-7b42597febdd" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_1aab9999-85bb-4a3c-a26d-c153b734bc4e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsRecurringMember_75ee960a-bb48-4251-8acd-7ab0c149a2f9" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueMeasurementsRecurringMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementFrequencyDomain_1aab9999-85bb-4a3c-a26d-c153b734bc4e" xlink:to="loc_us-gaap_FairValueMeasurementsRecurringMember_75ee960a-bb48-4251-8acd-7ab0c149a2f9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_bb993cd9-2f25-4452-bc70-e863913dcefa" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_0197a1c2-ffa1-404c-aa76-e8fb84554ab5" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_bb993cd9-2f25-4452-bc70-e863913dcefa" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_bb993cd9-2f25-4452-bc70-e863913dcefa_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_bb993cd9-2f25-4452-bc70-e863913dcefa" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_bb993cd9-2f25-4452-bc70-e863913dcefa_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_a542f08a-ec04-4ee9-bed2-2c0d3996aac9" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_bb993cd9-2f25-4452-bc70-e863913dcefa" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_a542f08a-ec04-4ee9-bed2-2c0d3996aac9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_5d8a6ab6-3ce1-4635-aadf-3620a2f5c586" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueInputsLevel1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_a542f08a-ec04-4ee9-bed2-2c0d3996aac9" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_5d8a6ab6-3ce1-4635-aadf-3620a2f5c586" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_8c896684-6fdf-414e-a7ac-3a965ca1962b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_a542f08a-ec04-4ee9-bed2-2c0d3996aac9" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_8c896684-6fdf-414e-a7ac-3a965ca1962b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_5de9f4a8-cc08-4fc3-8d4b-f38346e127ec" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_a542f08a-ec04-4ee9-bed2-2c0d3996aac9" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_5de9f4a8-cc08-4fc3-8d4b-f38346e127ec" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_a5c41b75-89a7-4824-a7d4-453811a194c5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_0197a1c2-ffa1-404c-aa76-e8fb84554ab5" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_a5c41b75-89a7-4824-a7d4-453811a194c5" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_a5c41b75-89a7-4824-a7d4-453811a194c5_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_a5c41b75-89a7-4824-a7d4-453811a194c5" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_a5c41b75-89a7-4824-a7d4-453811a194c5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_11d10fe9-f910-4a3a-8dd2-14ac91643489" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_a5c41b75-89a7-4824-a7d4-453811a194c5" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_11d10fe9-f910-4a3a-8dd2-14ac91643489" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LandfillMember_c1ade200-fafe-4906-a7bf-aa4d8dc2f89b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LandfillMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_11d10fe9-f910-4a3a-8dd2-14ac91643489" xlink:to="loc_us-gaap_LandfillMember_c1ade200-fafe-4906-a7bf-aa4d8dc2f89b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsAbstract_8029f56e-a626-4e06-94bb-e65daba2643c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AssetsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_bdb78d06-4c3a-4437-ba71-02a57b6a9e4b" xlink:to="loc_us-gaap_AssetsAbstract_8029f56e-a626-4e06-94bb-e65daba2643c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndInvestmentsNoncurrent_581bdfc7-fd00-43d5-b397-1af0926385ac" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RestrictedCashAndInvestmentsNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_8029f56e-a626-4e06-94bb-e65daba2643c" xlink:to="loc_us-gaap_RestrictedCashAndInvestmentsNoncurrent_581bdfc7-fd00-43d5-b397-1af0926385ac" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeAssets_f252b5de-8a36-4a4c-b8bb-67b619fd17e3" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DerivativeAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_8029f56e-a626-4e06-94bb-e65daba2643c" xlink:to="loc_us-gaap_DerivativeAssets_f252b5de-8a36-4a4c-b8bb-67b619fd17e3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_DerivativeAssetStatementOfFinancialPositionExtensibleEnumerationNotDisclosedFlag_67cf032e-9605-437a-a01b-8dc1066b491f" xlink:href="cwst-20250331.xsd#cwst_DerivativeAssetStatementOfFinancialPositionExtensibleEnumerationNotDisclosedFlag"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeAssets_f252b5de-8a36-4a4c-b8bb-67b619fd17e3" xlink:to="loc_cwst_DerivativeAssetStatementOfFinancialPositionExtensibleEnumerationNotDisclosedFlag_67cf032e-9605-437a-a01b-8dc1066b491f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsFairValueDisclosure_73b1f3bf-8859-4fd3-b047-e6bf99e7283c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AssetsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_8029f56e-a626-4e06-94bb-e65daba2643c" xlink:to="loc_us-gaap_AssetsFairValueDisclosure_73b1f3bf-8859-4fd3-b047-e6bf99e7283c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAbstract_fc9ad516-10f9-49aa-9831-4ba5e1cda9f7" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LiabilitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_bdb78d06-4c3a-4437-ba71-02a57b6a9e4b" xlink:to="loc_us-gaap_LiabilitiesAbstract_fc9ad516-10f9-49aa-9831-4ba5e1cda9f7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestRateDerivativeLiabilitiesAtFairValue_52988474-e472-4c20-bf98-28e8c2305fea" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_InterestRateDerivativeLiabilitiesAtFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAbstract_fc9ad516-10f9-49aa-9831-4ba5e1cda9f7" xlink:to="loc_us-gaap_InterestRateDerivativeLiabilitiesAtFairValue_52988474-e472-4c20-bf98-28e8c2305fea" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/FAIRVALUEOFFINANCIALINSTRUMENTSNarrativeDetail" xlink:type="simple" xlink:href="cwst-20250331.xsd#FAIRVALUEOFFINANCIALINSTRUMENTSNarrativeDetail"/>
  <link:definitionLink xlink:role="http://casella.com/role/FAIRVALUEOFFINANCIALINSTRUMENTSNarrativeDetail" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_752c4c0b-2971-485a-9fbd-3cbacfd08540" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_4eb66bf7-74e3-4e48-9b38-54bf3cd85bf9" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_752c4c0b-2971-485a-9fbd-3cbacfd08540" xlink:to="loc_us-gaap_DebtInstrumentTable_4eb66bf7-74e3-4e48-9b38-54bf3cd85bf9" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementBasisAxis_c5172a89-bb8d-44d4-ab7d-d2523900d4e9" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueByMeasurementBasisAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_4eb66bf7-74e3-4e48-9b38-54bf3cd85bf9" xlink:to="loc_us-gaap_FairValueByMeasurementBasisAxis_c5172a89-bb8d-44d4-ab7d-d2523900d4e9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_c5172a89-bb8d-44d4-ab7d-d2523900d4e9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementBasisAxis_c5172a89-bb8d-44d4-ab7d-d2523900d4e9" xlink:to="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_c5172a89-bb8d-44d4-ab7d-d2523900d4e9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueDisclosureItemAmountsDomain_9d00cc3e-b666-456e-b6d5-1260a77d1202" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueDisclosureItemAmountsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementBasisAxis_c5172a89-bb8d-44d4-ab7d-d2523900d4e9" xlink:to="loc_us-gaap_FairValueDisclosureItemAmountsDomain_9d00cc3e-b666-456e-b6d5-1260a77d1202" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_a352db1d-22ca-4ee4-8069-91365d94e70a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureItemAmountsDomain_9d00cc3e-b666-456e-b6d5-1260a77d1202" xlink:to="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_a352db1d-22ca-4ee4-8069-91365d94e70a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EstimateOfFairValueFairValueDisclosureMember_f62d0532-264b-4846-818f-515413e96345" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EstimateOfFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_a352db1d-22ca-4ee4-8069-91365d94e70a" xlink:to="loc_us-gaap_EstimateOfFairValueFairValueDisclosureMember_f62d0532-264b-4846-818f-515413e96345" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CarryingReportedAmountFairValueDisclosureMember_5ef63a6e-91e6-49b1-83f2-d2986c121aa3" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CarryingReportedAmountFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_a352db1d-22ca-4ee4-8069-91365d94e70a" xlink:to="loc_us-gaap_CarryingReportedAmountFairValueDisclosureMember_5ef63a6e-91e6-49b1-83f2-d2986c121aa3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_3ace6b2d-d717-4465-bfd8-7bb38d2ed8f6" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_4eb66bf7-74e3-4e48-9b38-54bf3cd85bf9" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_3ace6b2d-d717-4465-bfd8-7bb38d2ed8f6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_3ace6b2d-d717-4465-bfd8-7bb38d2ed8f6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_3ace6b2d-d717-4465-bfd8-7bb38d2ed8f6" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_3ace6b2d-d717-4465-bfd8-7bb38d2ed8f6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_4318920b-94f0-44b5-b864-ea3696923849" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_3ace6b2d-d717-4465-bfd8-7bb38d2ed8f6" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_4318920b-94f0-44b5-b864-ea3696923849" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_FixedRateBondsMember_fe2d54ff-c39a-4ca4-b984-39fe18dc9220" xlink:href="cwst-20250331.xsd#cwst_FixedRateBondsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_4318920b-94f0-44b5-b864-ea3696923849" xlink:to="loc_cwst_FixedRateBondsMember_fe2d54ff-c39a-4ca4-b984-39fe18dc9220" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_438527e4-54b3-4c24-bcac-bc27f26b956d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_4318920b-94f0-44b5-b864-ea3696923849" xlink:to="loc_us-gaap_SecuredDebtMember_438527e4-54b3-4c24-bcac-bc27f26b956d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_9b5f5f1b-b462-418d-9384-e7bad2852775" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_4318920b-94f0-44b5-b864-ea3696923849" xlink:to="loc_us-gaap_LineOfCreditMember_9b5f5f1b-b462-418d-9384-e7bad2852775" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFairValue_09749f94-90a9-4488-9745-2ea57deac01f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_752c4c0b-2971-485a-9fbd-3cbacfd08540" xlink:to="loc_us-gaap_DebtInstrumentFairValue_09749f94-90a9-4488-9745-2ea57deac01f" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/SEGMENTREPORTINGNarrativeDetails" xlink:type="simple" xlink:href="cwst-20250331.xsd#SEGMENTREPORTINGNarrativeDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/SEGMENTREPORTINGNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_23ba87b6-7a97-4b0e-ab58-9b1e8dd2b3de" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_0b7e2791-ba97-485f-b240-6687b23031bf" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_23ba87b6-7a97-4b0e-ab58-9b1e8dd2b3de" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_0b7e2791-ba97-485f-b240-6687b23031bf" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_d27be90a-1c5b-49e5-9ba2-0f1280285b63" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_0b7e2791-ba97-485f-b240-6687b23031bf" xlink:to="loc_srt_ProductOrServiceAxis_d27be90a-1c5b-49e5-9ba2-0f1280285b63" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_d27be90a-1c5b-49e5-9ba2-0f1280285b63_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_d27be90a-1c5b-49e5-9ba2-0f1280285b63" xlink:to="loc_srt_ProductsAndServicesDomain_d27be90a-1c5b-49e5-9ba2-0f1280285b63_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_5932adf3-1120-4418-98a2-d42d4cc42e21" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_d27be90a-1c5b-49e5-9ba2-0f1280285b63" xlink:to="loc_srt_ProductsAndServicesDomain_5932adf3-1120-4418-98a2-d42d4cc42e21" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_SolidWasteOperationsMember_8af27693-8cf1-4ec3-9517-fdbcb7e9c3eb" xlink:href="cwst-20250331.xsd#cwst_SolidWasteOperationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_5932adf3-1120-4418-98a2-d42d4cc42e21" xlink:to="loc_cwst_SolidWasteOperationsMember_8af27693-8cf1-4ec3-9517-fdbcb7e9c3eb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfOperatingSegments_fb3a352d-322d-4ec2-a0fa-a81abc348e78" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_NumberOfOperatingSegments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_23ba87b6-7a97-4b0e-ab58-9b1e8dd2b3de" xlink:to="loc_us-gaap_NumberOfOperatingSegments_fb3a352d-322d-4ec2-a0fa-a81abc348e78" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/SEGMENTREPORTINGSummaryofFinancialInformationbyReportableSegmentDetail" xlink:type="simple" xlink:href="cwst-20250331.xsd#SEGMENTREPORTINGSummaryofFinancialInformationbyReportableSegmentDetail"/>
  <link:definitionLink xlink:role="http://casella.com/role/SEGMENTREPORTINGSummaryofFinancialInformationbyReportableSegmentDetail" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_7f81f93a-a118-4a76-a2f1-2443e340b0ab" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_15982058-2e52-4311-bdb8-15110c353a40" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_7f81f93a-a118-4a76-a2f1-2443e340b0ab" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_15982058-2e52-4311-bdb8-15110c353a40" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsAxis_ed367fbe-4e20-4db3-b88d-4bba83b5232f" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ConsolidationItemsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_15982058-2e52-4311-bdb8-15110c353a40" xlink:to="loc_srt_ConsolidationItemsAxis_ed367fbe-4e20-4db3-b88d-4bba83b5232f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_ed367fbe-4e20-4db3-b88d-4bba83b5232f_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidationItemsAxis_ed367fbe-4e20-4db3-b88d-4bba83b5232f" xlink:to="loc_srt_ConsolidationItemsDomain_ed367fbe-4e20-4db3-b88d-4bba83b5232f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_8576615b-0c9b-4507-9e9f-af4f272113ff" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidationItemsAxis_ed367fbe-4e20-4db3-b88d-4bba83b5232f" xlink:to="loc_srt_ConsolidationItemsDomain_8576615b-0c9b-4507-9e9f-af4f272113ff" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IntersegmentEliminationMember_3b122655-a235-45dd-8a39-5392b8a3af4b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IntersegmentEliminationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_8576615b-0c9b-4507-9e9f-af4f272113ff" xlink:to="loc_us-gaap_IntersegmentEliminationMember_3b122655-a235-45dd-8a39-5392b8a3af4b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingSegmentsMember_2299577f-99b8-4d46-a838-8a36a0e9187b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OperatingSegmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_8576615b-0c9b-4507-9e9f-af4f272113ff" xlink:to="loc_us-gaap_OperatingSegmentsMember_2299577f-99b8-4d46-a838-8a36a0e9187b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateNonSegmentMember_de76ec69-1236-4c90-9757-930d64fbf743" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CorporateNonSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_8576615b-0c9b-4507-9e9f-af4f272113ff" xlink:to="loc_us-gaap_CorporateNonSegmentMember_de76ec69-1236-4c90-9757-930d64fbf743" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_e1ab9ae0-b152-471b-b110-665bb56ccd87" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_15982058-2e52-4311-bdb8-15110c353a40" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_e1ab9ae0-b152-471b-b110-665bb56ccd87" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_e1ab9ae0-b152-471b-b110-665bb56ccd87_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_e1ab9ae0-b152-471b-b110-665bb56ccd87" xlink:to="loc_us-gaap_SegmentDomain_e1ab9ae0-b152-471b-b110-665bb56ccd87_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_e1218519-8db0-45f2-a9c2-7e5a7050c7ad" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_e1ab9ae0-b152-471b-b110-665bb56ccd87" xlink:to="loc_us-gaap_SegmentDomain_e1218519-8db0-45f2-a9c2-7e5a7050c7ad" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_SolidWasteOperationsMember_ebe54e87-f86d-45f0-ae34-1de2c5ed9609" xlink:href="cwst-20250331.xsd#cwst_SolidWasteOperationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_e1218519-8db0-45f2-a9c2-7e5a7050c7ad" xlink:to="loc_cwst_SolidWasteOperationsMember_ebe54e87-f86d-45f0-ae34-1de2c5ed9609" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_ResourceSolutionsRegionMember_8866f392-b7a7-4d75-8a54-4b060415ea79" xlink:href="cwst-20250331.xsd#cwst_ResourceSolutionsRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_e1218519-8db0-45f2-a9c2-7e5a7050c7ad" xlink:to="loc_cwst_ResourceSolutionsRegionMember_8866f392-b7a7-4d75-8a54-4b060415ea79" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsegmentsAxis_9df55b5a-fbd4-4866-aa68-7a4ecee4663a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SubsegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_15982058-2e52-4311-bdb8-15110c353a40" xlink:to="loc_us-gaap_SubsegmentsAxis_9df55b5a-fbd4-4866-aa68-7a4ecee4663a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsegmentsDomain_9df55b5a-fbd4-4866-aa68-7a4ecee4663a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SubsegmentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsegmentsAxis_9df55b5a-fbd4-4866-aa68-7a4ecee4663a" xlink:to="loc_us-gaap_SubsegmentsDomain_9df55b5a-fbd4-4866-aa68-7a4ecee4663a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsegmentsDomain_9bb27f15-308e-47ff-94d3-e17367c22491" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SubsegmentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsegmentsAxis_9df55b5a-fbd4-4866-aa68-7a4ecee4663a" xlink:to="loc_us-gaap_SubsegmentsDomain_9bb27f15-308e-47ff-94d3-e17367c22491" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_EasternRegionMember_d70b98b6-7fa1-4cc9-8852-b56809d471ee" xlink:href="cwst-20250331.xsd#cwst_EasternRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsegmentsDomain_9bb27f15-308e-47ff-94d3-e17367c22491" xlink:to="loc_cwst_EasternRegionMember_d70b98b6-7fa1-4cc9-8852-b56809d471ee" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_WesternRegionMember_4e51307b-edb7-4d8e-83d8-43f04d36ef28" xlink:href="cwst-20250331.xsd#cwst_WesternRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsegmentsDomain_9bb27f15-308e-47ff-94d3-e17367c22491" xlink:to="loc_cwst_WesternRegionMember_4e51307b-edb7-4d8e-83d8-43f04d36ef28" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_MidAtlanticRegionMember_6ff42a9c-ff1c-4a42-b1db-d04a8341edd7" xlink:href="cwst-20250331.xsd#cwst_MidAtlanticRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsegmentsDomain_9bb27f15-308e-47ff-94d3-e17367c22491" xlink:to="loc_cwst_MidAtlanticRegionMember_6ff42a9c-ff1c-4a42-b1db-d04a8341edd7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Revenues_6b633034-118d-483f-8844-1ba07a7549a2" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_Revenues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_7f81f93a-a118-4a76-a2f1-2443e340b0ab" xlink:to="loc_us-gaap_Revenues_6b633034-118d-483f-8844-1ba07a7549a2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfGoodsAndServicesSold_2dad407c-ef8b-4727-9114-0fc10b3158b4" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CostOfGoodsAndServicesSold"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_7f81f93a-a118-4a76-a2f1-2443e340b0ab" xlink:to="loc_us-gaap_CostOfGoodsAndServicesSold_2dad407c-ef8b-4727-9114-0fc10b3158b4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GeneralAndAdministrativeExpense_1a01991f-dbda-40e8-aca7-0b47740608be" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_GeneralAndAdministrativeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_7f81f93a-a118-4a76-a2f1-2443e340b0ab" xlink:to="loc_us-gaap_GeneralAndAdministrativeExpense_1a01991f-dbda-40e8-aca7-0b47740608be" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DepreciationDepletionAndAmortization_cc1203bf-2213-427f-a6af-7f08b09b0d7d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DepreciationDepletionAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_7f81f93a-a118-4a76-a2f1-2443e340b0ab" xlink:to="loc_us-gaap_DepreciationDepletionAndAmortization_cc1203bf-2213-427f-a6af-7f08b09b0d7d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationAcquisitionRelatedCosts_fb8a31a0-4769-46fc-8094-1a93d43021db" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_BusinessCombinationAcquisitionRelatedCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_7f81f93a-a118-4a76-a2f1-2443e340b0ab" xlink:to="loc_us-gaap_BusinessCombinationAcquisitionRelatedCosts_fb8a31a0-4769-46fc-8094-1a93d43021db" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingIncomeLoss_62c4c0a9-0d2c-41f4-b0e7-1502d2db55c4" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OperatingIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_7f81f93a-a118-4a76-a2f1-2443e340b0ab" xlink:to="loc_us-gaap_OperatingIncomeLoss_62c4c0a9-0d2c-41f4-b0e7-1502d2db55c4" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestIncomeExpenseNonoperatingNet_5ff5097b-9143-44b1-9abf-3429c55e8430" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_InterestIncomeExpenseNonoperatingNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_7f81f93a-a118-4a76-a2f1-2443e340b0ab" xlink:to="loc_us-gaap_InterestIncomeExpenseNonoperatingNet_5ff5097b-9143-44b1-9abf-3429c55e8430" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNonoperatingIncomeExpense_afa5df67-d575-4841-9928-c337f55431a4" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherNonoperatingIncomeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_7f81f93a-a118-4a76-a2f1-2443e340b0ab" xlink:to="loc_us-gaap_OtherNonoperatingIncomeExpense_afa5df67-d575-4841-9928-c337f55431a4" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_c9489aff-9fef-487a-b40d-d6eb5b475a8b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_7f81f93a-a118-4a76-a2f1-2443e340b0ab" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_c9489aff-9fef-487a-b40d-d6eb5b475a8b" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentExpenditureAdditionToLongLivedAssets_36fa7e3a-867b-4dcc-a479-1e93b2e484b9" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SegmentExpenditureAdditionToLongLivedAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_7f81f93a-a118-4a76-a2f1-2443e340b0ab" xlink:to="loc_us-gaap_SegmentExpenditureAdditionToLongLivedAssets_36fa7e3a-867b-4dcc-a479-1e93b2e484b9" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Assets_f315d182-5592-4e25-ad44-3562d7856501" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_Assets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_7f81f93a-a118-4a76-a2f1-2443e340b0ab" xlink:to="loc_us-gaap_Assets_f315d182-5592-4e25-ad44-3562d7856501" xlink:type="arc" order="10"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/SEGMENTREPORTINGSummaryofRevenuesAttributabletoServicesProvidedDetail" xlink:type="simple" xlink:href="cwst-20250331.xsd#SEGMENTREPORTINGSummaryofRevenuesAttributabletoServicesProvidedDetail"/>
  <link:definitionLink xlink:role="http://casella.com/role/SEGMENTREPORTINGSummaryofRevenuesAttributabletoServicesProvidedDetail" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_9e5aa66e-70bf-4deb-9558-0c514e5b3019" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_047f0939-f650-4079-a684-13a87b60c5b1" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_9e5aa66e-70bf-4deb-9558-0c514e5b3019" xlink:to="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_047f0939-f650-4079-a684-13a87b60c5b1" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_f28ba271-219a-4b90-a2ad-5fc06fa4efa2" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_047f0939-f650-4079-a684-13a87b60c5b1" xlink:to="loc_srt_ProductOrServiceAxis_f28ba271-219a-4b90-a2ad-5fc06fa4efa2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_f28ba271-219a-4b90-a2ad-5fc06fa4efa2_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_f28ba271-219a-4b90-a2ad-5fc06fa4efa2" xlink:to="loc_srt_ProductsAndServicesDomain_f28ba271-219a-4b90-a2ad-5fc06fa4efa2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_334d5f92-f995-420b-bae5-bd1b04e9184e" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_f28ba271-219a-4b90-a2ad-5fc06fa4efa2" xlink:to="loc_srt_ProductsAndServicesDomain_334d5f92-f995-420b-bae5-bd1b04e9184e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_SolidWasteOperationsMember_32633e46-2b56-440f-8327-30ca862a3dc7" xlink:href="cwst-20250331.xsd#cwst_SolidWasteOperationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_334d5f92-f995-420b-bae5-bd1b04e9184e" xlink:to="loc_cwst_SolidWasteOperationsMember_32633e46-2b56-440f-8327-30ca862a3dc7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_SolidWasteOperationsCollectionMember_82675006-fe9e-488a-8c73-d2f32deae952" xlink:href="cwst-20250331.xsd#cwst_SolidWasteOperationsCollectionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cwst_SolidWasteOperationsMember_32633e46-2b56-440f-8327-30ca862a3dc7" xlink:to="loc_cwst_SolidWasteOperationsCollectionMember_82675006-fe9e-488a-8c73-d2f32deae952" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_SolidWasteOperationsDisposalMember_4477dec9-1adc-4a06-8828-8d3878daf829" xlink:href="cwst-20250331.xsd#cwst_SolidWasteOperationsDisposalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cwst_SolidWasteOperationsMember_32633e46-2b56-440f-8327-30ca862a3dc7" xlink:to="loc_cwst_SolidWasteOperationsDisposalMember_4477dec9-1adc-4a06-8828-8d3878daf829" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_SolidWasteOperationsLandfillGasToEnergyMember_e46d4139-0c68-4cae-bbaf-4763d5a69f17" xlink:href="cwst-20250331.xsd#cwst_SolidWasteOperationsLandfillGasToEnergyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cwst_SolidWasteOperationsMember_32633e46-2b56-440f-8327-30ca862a3dc7" xlink:to="loc_cwst_SolidWasteOperationsLandfillGasToEnergyMember_e46d4139-0c68-4cae-bbaf-4763d5a69f17" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_SolidWasteOperationsProcessingMember_1cd05847-3926-4e04-8b40-8da9ef40c40e" xlink:href="cwst-20250331.xsd#cwst_SolidWasteOperationsProcessingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cwst_SolidWasteOperationsMember_32633e46-2b56-440f-8327-30ca862a3dc7" xlink:to="loc_cwst_SolidWasteOperationsProcessingMember_1cd05847-3926-4e04-8b40-8da9ef40c40e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_ResourceSolutionsOperationsMember_f7c32d0c-640b-47fa-aac7-7a0b46f8d1a0" xlink:href="cwst-20250331.xsd#cwst_ResourceSolutionsOperationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_334d5f92-f995-420b-bae5-bd1b04e9184e" xlink:to="loc_cwst_ResourceSolutionsOperationsMember_f7c32d0c-640b-47fa-aac7-7a0b46f8d1a0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_ResourceSolutionsOperationsProcessingMember_e163b0d7-23b5-40d8-8287-5ec6e8bac614" xlink:href="cwst-20250331.xsd#cwst_ResourceSolutionsOperationsProcessingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cwst_ResourceSolutionsOperationsMember_f7c32d0c-640b-47fa-aac7-7a0b46f8d1a0" xlink:to="loc_cwst_ResourceSolutionsOperationsProcessingMember_e163b0d7-23b5-40d8-8287-5ec6e8bac614" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_NationalAccountsMember_f4b57f16-9f83-4521-9803-1fc2a9f1b584" xlink:href="cwst-20250331.xsd#cwst_NationalAccountsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cwst_ResourceSolutionsOperationsMember_f7c32d0c-640b-47fa-aac7-7a0b46f8d1a0" xlink:to="loc_cwst_NationalAccountsMember_f4b57f16-9f83-4521-9803-1fc2a9f1b584" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Revenues_5231e8af-a4fd-4299-8096-8b06e0637805" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_Revenues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_9e5aa66e-70bf-4deb-9558-0c514e5b3019" xlink:to="loc_us-gaap_Revenues_5231e8af-a4fd-4299-8096-8b06e0637805" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fasb.org/us-gaap/role/eedm/ExtensibleEnumerationLists" xlink:type="simple" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-roles-2024.xsd#eedm"/>
  <link:definitionLink xlink:role="http://fasb.org/us-gaap/role/eedm/ExtensibleEnumerationLists" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAbstract" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LiabilitiesAbstract"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtAndCapitalLeaseObligations" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LongTermDebtAndCapitalLeaseObligations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAbstract" xlink:to="loc_us-gaap_LongTermDebtAndCapitalLeaseObligations" xlink:type="arc" order="31"/>
  </link:definitionLink>
</link:linkbase>
