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

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:019ce26e-a108-7f9f-9c9a-a9a9ae784ce0,g:ed223d8c-adef-4a39-8e98-88b8a5b03f0b-->
<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-20260331.xsd#COVER"/>
  <link:definitionLink xlink:role="http://casella.com/role/COVER" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentInformationLineItems_019ce26e-a34e-748f-96c9-39b54d0f228b" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentInformationTable_019ce26e-a34e-729b-9ab4-e905e14ecc5c" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentInformationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_dei_DocumentInformationLineItems_019ce26e-a34e-748f-96c9-39b54d0f228b" xlink:to="loc_dei_DocumentInformationTable_019ce26e-a34e-729b-9ab4-e905e14ecc5c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_019ce26e-a34e-75c5-ae2a-6b13630309e9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dei_DocumentInformationTable_019ce26e-a34e-729b-9ab4-e905e14ecc5c" xlink:to="loc_us-gaap_StatementClassOfStockAxis_019ce26e-a34e-75c5-ae2a-6b13630309e9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019ce26e-a34e-75c5-ae2a-6b13630309e9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019ce26e-a34e-75c5-ae2a-6b13630309e9" xlink:to="loc_us-gaap_ClassOfStockDomain_019ce26e-a34e-75c5-ae2a-6b13630309e9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019ce26e-a34e-7b76-895b-ee04bc4b545b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019ce26e-a34e-75c5-ae2a-6b13630309e9" xlink:to="loc_us-gaap_ClassOfStockDomain_019ce26e-a34e-7b76-895b-ee04bc4b545b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_019ce26e-a34e-705b-9130-a01591be8aef" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019ce26e-a34e-7b76-895b-ee04bc4b545b" xlink:to="loc_us-gaap_CommonClassAMember_019ce26e-a34e-705b-9130-a01591be8aef" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_019ce26e-a34e-7796-919b-5cfe4e4ba830" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019ce26e-a34e-7b76-895b-ee04bc4b545b" xlink:to="loc_us-gaap_CommonClassBMember_019ce26e-a34e-7796-919b-5cfe4e4ba830" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentType_019ce26e-a34e-7a57-80cd-ec375c2b3eb1" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentType"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019ce26e-a34e-748f-96c9-39b54d0f228b" xlink:to="loc_dei_DocumentType_019ce26e-a34e-7a57-80cd-ec375c2b3eb1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentQuarterlyReport_019ce26e-a34e-766e-b2b0-6d565c4adca4" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentQuarterlyReport"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019ce26e-a34e-748f-96c9-39b54d0f228b" xlink:to="loc_dei_DocumentQuarterlyReport_019ce26e-a34e-766e-b2b0-6d565c4adca4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentPeriodEndDate_019ce26e-a34e-7ea0-9aa8-220ad334d92d" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentPeriodEndDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019ce26e-a34e-748f-96c9-39b54d0f228b" xlink:to="loc_dei_DocumentPeriodEndDate_019ce26e-a34e-7ea0-9aa8-220ad334d92d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentTransitionReport_019ce26e-a34e-7e1f-8e5a-8468943de49f" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentTransitionReport"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019ce26e-a34e-748f-96c9-39b54d0f228b" xlink:to="loc_dei_DocumentTransitionReport_019ce26e-a34e-7e1f-8e5a-8468943de49f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFileNumber_019ce26e-a34e-7d3c-bfef-a51196811ab9" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityFileNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019ce26e-a34e-748f-96c9-39b54d0f228b" xlink:to="loc_dei_EntityFileNumber_019ce26e-a34e-7d3c-bfef-a51196811ab9" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityRegistrantName_019ce26e-a34e-7846-b28d-ebc635835c32" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityRegistrantName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019ce26e-a34e-748f-96c9-39b54d0f228b" xlink:to="loc_dei_EntityRegistrantName_019ce26e-a34e-7846-b28d-ebc635835c32" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityIncorporationStateCountryCode_019ce26e-a34e-77b5-97da-b4eaed028b20" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityIncorporationStateCountryCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019ce26e-a34e-748f-96c9-39b54d0f228b" xlink:to="loc_dei_EntityIncorporationStateCountryCode_019ce26e-a34e-77b5-97da-b4eaed028b20" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityTaxIdentificationNumber_019ce26e-a34e-778e-9b72-bf81ce1cae35" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityTaxIdentificationNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019ce26e-a34e-748f-96c9-39b54d0f228b" xlink:to="loc_dei_EntityTaxIdentificationNumber_019ce26e-a34e-778e-9b72-bf81ce1cae35" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressAddressLine1_019ce26e-a34e-72b6-890b-f5e49f586f47" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityAddressAddressLine1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019ce26e-a34e-748f-96c9-39b54d0f228b" xlink:to="loc_dei_EntityAddressAddressLine1_019ce26e-a34e-72b6-890b-f5e49f586f47" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressCityOrTown_019ce26e-a34e-7730-9dc3-15f00a8ebe6e" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityAddressCityOrTown"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019ce26e-a34e-748f-96c9-39b54d0f228b" xlink:to="loc_dei_EntityAddressCityOrTown_019ce26e-a34e-7730-9dc3-15f00a8ebe6e" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressStateOrProvince_019ce26e-a34e-7880-8482-85a4e7d82bb9" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityAddressStateOrProvince"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019ce26e-a34e-748f-96c9-39b54d0f228b" xlink:to="loc_dei_EntityAddressStateOrProvince_019ce26e-a34e-7880-8482-85a4e7d82bb9" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressPostalZipCode_019ce26e-a34e-73d2-a4e8-4710ab4f6c28" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityAddressPostalZipCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019ce26e-a34e-748f-96c9-39b54d0f228b" xlink:to="loc_dei_EntityAddressPostalZipCode_019ce26e-a34e-73d2-a4e8-4710ab4f6c28" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CityAreaCode_019ce26e-a34e-7786-85d0-9749a37384e3" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_CityAreaCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019ce26e-a34e-748f-96c9-39b54d0f228b" xlink:to="loc_dei_CityAreaCode_019ce26e-a34e-7786-85d0-9749a37384e3" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LocalPhoneNumber_019ce26e-a34e-75f8-91cf-e9ab3e0c78e3" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LocalPhoneNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019ce26e-a34e-748f-96c9-39b54d0f228b" xlink:to="loc_dei_LocalPhoneNumber_019ce26e-a34e-75f8-91cf-e9ab3e0c78e3" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_Security12bTitle_019ce26e-a34e-7e0e-896b-50468f452dd2" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_Security12bTitle"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019ce26e-a34e-748f-96c9-39b54d0f228b" xlink:to="loc_dei_Security12bTitle_019ce26e-a34e-7e0e-896b-50468f452dd2" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_TradingSymbol_019ce26e-a34e-7fc5-a91d-f3bd7e3d8a93" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_TradingSymbol"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019ce26e-a34e-748f-96c9-39b54d0f228b" xlink:to="loc_dei_TradingSymbol_019ce26e-a34e-7fc5-a91d-f3bd7e3d8a93" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_SecurityExchangeName_019ce26e-a34e-7d06-9434-b118135e8061" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_SecurityExchangeName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019ce26e-a34e-748f-96c9-39b54d0f228b" xlink:to="loc_dei_SecurityExchangeName_019ce26e-a34e-7d06-9434-b118135e8061" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCurrentReportingStatus_019ce26e-a34e-7bef-b2c0-86d2ecb6d622" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityCurrentReportingStatus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019ce26e-a34e-748f-96c9-39b54d0f228b" xlink:to="loc_dei_EntityCurrentReportingStatus_019ce26e-a34e-7bef-b2c0-86d2ecb6d622" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityInteractiveDataCurrent_019ce26e-a34e-7d73-a857-1d984642a877" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityInteractiveDataCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019ce26e-a34e-748f-96c9-39b54d0f228b" xlink:to="loc_dei_EntityInteractiveDataCurrent_019ce26e-a34e-7d73-a857-1d984642a877" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFilerCategory_019ce26e-a34e-72fe-a0a9-2c191cebc3f6" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityFilerCategory"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019ce26e-a34e-748f-96c9-39b54d0f228b" xlink:to="loc_dei_EntityFilerCategory_019ce26e-a34e-72fe-a0a9-2c191cebc3f6" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntitySmallBusiness_019ce26e-a34e-7991-af2c-f5f7361f1dd0" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntitySmallBusiness"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019ce26e-a34e-748f-96c9-39b54d0f228b" xlink:to="loc_dei_EntitySmallBusiness_019ce26e-a34e-7991-af2c-f5f7361f1dd0" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityEmergingGrowthCompany_019ce26e-a34e-7aa9-b1ee-e4b4343106e0" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityEmergingGrowthCompany"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019ce26e-a34e-748f-96c9-39b54d0f228b" xlink:to="loc_dei_EntityEmergingGrowthCompany_019ce26e-a34e-7aa9-b1ee-e4b4343106e0" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityShellCompany_019ce26e-a34e-72f8-92db-ea3478bec5c6" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityShellCompany"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019ce26e-a34e-748f-96c9-39b54d0f228b" xlink:to="loc_dei_EntityShellCompany_019ce26e-a34e-72f8-92db-ea3478bec5c6" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCommonStockSharesOutstanding_019ce26e-a34e-7a68-b7b5-5925f049e44c" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityCommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019ce26e-a34e-748f-96c9-39b54d0f228b" xlink:to="loc_dei_EntityCommonStockSharesOutstanding_019ce26e-a34e-7a68-b7b5-5925f049e44c" xlink:type="arc" order="23"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCentralIndexKey_019ce26e-a34e-7983-aea7-70360f5338a9" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityCentralIndexKey"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019ce26e-a34e-748f-96c9-39b54d0f228b" xlink:to="loc_dei_EntityCentralIndexKey_019ce26e-a34e-7983-aea7-70360f5338a9" xlink:type="arc" order="24"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CurrentFiscalYearEndDate_019ce26e-a34e-7fdd-a804-4ddc3faba5df" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_CurrentFiscalYearEndDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019ce26e-a34e-748f-96c9-39b54d0f228b" xlink:to="loc_dei_CurrentFiscalYearEndDate_019ce26e-a34e-7fdd-a804-4ddc3faba5df" xlink:type="arc" order="25"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentFiscalYearFocus_019ce26e-a34e-796f-a32b-6ca86174f854" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentFiscalYearFocus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019ce26e-a34e-748f-96c9-39b54d0f228b" xlink:to="loc_dei_DocumentFiscalYearFocus_019ce26e-a34e-796f-a32b-6ca86174f854" xlink:type="arc" order="26"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentFiscalPeriodFocus_019ce26e-a34e-75b3-95b9-e8c4716842c7" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentFiscalPeriodFocus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019ce26e-a34e-748f-96c9-39b54d0f228b" xlink:to="loc_dei_DocumentFiscalPeriodFocus_019ce26e-a34e-75b3-95b9-e8c4716842c7" xlink:type="arc" order="27"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_AmendmentFlag_019ce26e-a34e-77b6-807d-46724ff73d5f" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_AmendmentFlag"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019ce26e-a34e-748f-96c9-39b54d0f228b" xlink:to="loc_dei_AmendmentFlag_019ce26e-a34e-77b6-807d-46724ff73d5f" xlink:type="arc" order="28"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/CONSOLIDATEDBALANCESHEETS" xlink:type="simple" xlink:href="cwst-20260331.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_019ce26e-a34e-77a9-8313-8d82938aaec5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019ce26e-a34e-761a-9adc-9ecdb7d02463" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019ce26e-a34e-77a9-8313-8d82938aaec5" xlink:to="loc_us-gaap_StatementTable_019ce26e-a34e-761a-9adc-9ecdb7d02463" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_019ce26e-a34e-796b-81d1-7c9bfc2071c0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019ce26e-a34e-761a-9adc-9ecdb7d02463" xlink:to="loc_us-gaap_StatementClassOfStockAxis_019ce26e-a34e-796b-81d1-7c9bfc2071c0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019ce26e-a34e-796b-81d1-7c9bfc2071c0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019ce26e-a34e-796b-81d1-7c9bfc2071c0" xlink:to="loc_us-gaap_ClassOfStockDomain_019ce26e-a34e-796b-81d1-7c9bfc2071c0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019ce26e-a34e-797e-9ea9-c6b474c42a5f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019ce26e-a34e-796b-81d1-7c9bfc2071c0" xlink:to="loc_us-gaap_ClassOfStockDomain_019ce26e-a34e-797e-9ea9-c6b474c42a5f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_019ce26e-a34e-73cf-baa6-d1e6e9d390c8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019ce26e-a34e-797e-9ea9-c6b474c42a5f" xlink:to="loc_us-gaap_CommonClassAMember_019ce26e-a34e-73cf-baa6-d1e6e9d390c8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_019ce26e-a34e-796e-9260-aa7e9c13d68d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019ce26e-a34e-797e-9ea9-c6b474c42a5f" xlink:to="loc_us-gaap_CommonClassBMember_019ce26e-a34e-796e-9260-aa7e9c13d68d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsAbstract_019ce26e-a34e-7ba8-980c-ad008e0b0b1e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019ce26e-a34e-77a9-8313-8d82938aaec5" xlink:to="loc_us-gaap_AssetsAbstract_019ce26e-a34e-7ba8-980c-ad008e0b0b1e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsCurrentAbstract_019ce26e-a34e-76d0-8ccf-b62d3aecb76b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsCurrentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019ce26e-a34e-7ba8-980c-ad008e0b0b1e" xlink:to="loc_us-gaap_AssetsCurrentAbstract_019ce26e-a34e-76d0-8ccf-b62d3aecb76b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_019ce26e-a34e-78f1-a9c8-cb75c82a1412" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_019ce26e-a34e-76d0-8ccf-b62d3aecb76b" xlink:to="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_019ce26e-a34e-78f1-a9c8-cb75c82a1412" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableNetCurrent_019ce26e-a34e-70fc-8b97-2283cf5098ad" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccountsReceivableNetCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_019ce26e-a34e-76d0-8ccf-b62d3aecb76b" xlink:to="loc_us-gaap_AccountsReceivableNetCurrent_019ce26e-a34e-70fc-8b97-2283cf5098ad" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrepaidExpenseCurrent_019ce26e-a34e-74df-9e5b-9d038dba56f0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PrepaidExpenseCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_019ce26e-a34e-76d0-8ccf-b62d3aecb76b" xlink:to="loc_us-gaap_PrepaidExpenseCurrent_019ce26e-a34e-74df-9e5b-9d038dba56f0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherAssetsCurrent_019ce26e-a34f-7c30-ba8f-2d2061e3859e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherAssetsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_019ce26e-a34e-76d0-8ccf-b62d3aecb76b" xlink:to="loc_us-gaap_OtherAssetsCurrent_019ce26e-a34f-7c30-ba8f-2d2061e3859e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsCurrent_019ce26e-a34f-78d6-bdcd-3b744680f1d9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_019ce26e-a34e-76d0-8ccf-b62d3aecb76b" xlink:to="loc_us-gaap_AssetsCurrent_019ce26e-a34f-78d6-bdcd-3b744680f1d9" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization_019ce26e-a34f-710b-9a98-cecf64deb238" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019ce26e-a34e-7ba8-980c-ad008e0b0b1e" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization_019ce26e-a34f-710b-9a98-cecf64deb238" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseRightOfUseAsset_019ce26e-a34f-7ca8-97ac-38440b34b4a0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseRightOfUseAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019ce26e-a34e-7ba8-980c-ad008e0b0b1e" xlink:to="loc_us-gaap_OperatingLeaseRightOfUseAsset_019ce26e-a34f-7ca8-97ac-38440b34b4a0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_019ce26e-a34f-71c9-8867-46e51391b84f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019ce26e-a34e-7ba8-980c-ad008e0b0b1e" xlink:to="loc_us-gaap_Goodwill_019ce26e-a34f-71c9-8867-46e51391b84f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsNet_019ce26e-a34f-7edb-a38f-f0d372d3ce49" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019ce26e-a34e-7ba8-980c-ad008e0b0b1e" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsNet_019ce26e-a34f-7edb-a38f-f0d372d3ce49" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndInvestmentsNoncurrent_019ce26e-a34f-71dc-88b6-21500f97bcad" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedCashAndInvestmentsNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019ce26e-a34e-7ba8-980c-ad008e0b0b1e" xlink:to="loc_us-gaap_RestrictedCashAndInvestmentsNoncurrent_019ce26e-a34f-71dc-88b6-21500f97bcad" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherInvestmentsAndSecuritiesAtCost_019ce26e-a34f-711e-82d6-e6b7757e1325" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherInvestmentsAndSecuritiesAtCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019ce26e-a34e-7ba8-980c-ad008e0b0b1e" xlink:to="loc_us-gaap_OtherInvestmentsAndSecuritiesAtCost_019ce26e-a34f-711e-82d6-e6b7757e1325" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherAssetsNoncurrent_019ce26e-a34f-7a0e-a199-6c1d94bdabe5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherAssetsNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019ce26e-a34e-7ba8-980c-ad008e0b0b1e" xlink:to="loc_us-gaap_OtherAssetsNoncurrent_019ce26e-a34f-7a0e-a199-6c1d94bdabe5" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Assets_019ce26e-a34f-7c63-a627-14d3c4cbbbd8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Assets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019ce26e-a34e-7ba8-980c-ad008e0b0b1e" xlink:to="loc_us-gaap_Assets_019ce26e-a34f-7c63-a627-14d3c4cbbbd8" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_019ce26e-a34f-7ece-8509-add4f885ace4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesAndStockholdersEquityAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019ce26e-a34e-77a9-8313-8d82938aaec5" xlink:to="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_019ce26e-a34f-7ece-8509-add4f885ace4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesCurrentAbstract_019ce26e-a34f-7128-8961-3efaf06ac90d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesCurrentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_019ce26e-a34f-7ece-8509-add4f885ace4" xlink:to="loc_us-gaap_LiabilitiesCurrentAbstract_019ce26e-a34f-7128-8961-3efaf06ac90d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtAndCapitalLeaseObligationsCurrent_019ce26e-a34f-7ab1-9cef-5698b0753644" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtAndCapitalLeaseObligationsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_019ce26e-a34f-7128-8961-3efaf06ac90d" xlink:to="loc_us-gaap_LongTermDebtAndCapitalLeaseObligationsCurrent_019ce26e-a34f-7ab1-9cef-5698b0753644" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseLiabilityCurrent_019ce26e-a34f-75f9-ad97-549360885eef" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_019ce26e-a34f-7128-8961-3efaf06ac90d" xlink:to="loc_us-gaap_OperatingLeaseLiabilityCurrent_019ce26e-a34f-75f9-ad97-549360885eef" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsPayableCurrent_019ce26e-a34f-751f-82b1-fed3161e6bd0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccountsPayableCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_019ce26e-a34f-7128-8961-3efaf06ac90d" xlink:to="loc_us-gaap_AccountsPayableCurrent_019ce26e-a34f-751f-82b1-fed3161e6bd0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeRelatedLiabilitiesCurrent_019ce26e-a34f-7ec6-891f-57bd6cad21f0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeRelatedLiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_019ce26e-a34f-7128-8961-3efaf06ac90d" xlink:to="loc_us-gaap_EmployeeRelatedLiabilitiesCurrent_019ce26e-a34f-7ec6-891f-57bd6cad21f0" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityCurrent_019ce26e-a34f-7dbe-b388-ebfdfa5bb084" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ContractWithCustomerLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_019ce26e-a34f-7128-8961-3efaf06ac90d" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityCurrent_019ce26e-a34f-7dbe-b388-ebfdfa5bb084" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccruedCappingClosurePostClosureAndEnvironmentalCosts_019ce26e-a34f-758b-96d3-77f37df5180d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccruedCappingClosurePostClosureAndEnvironmentalCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_019ce26e-a34f-7128-8961-3efaf06ac90d" xlink:to="loc_us-gaap_AccruedCappingClosurePostClosureAndEnvironmentalCosts_019ce26e-a34f-758b-96d3-77f37df5180d" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherAccruedLiabilitiesCurrent_019ce26e-a34f-7299-9568-0f8eea62e95a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherAccruedLiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_019ce26e-a34f-7128-8961-3efaf06ac90d" xlink:to="loc_us-gaap_OtherAccruedLiabilitiesCurrent_019ce26e-a34f-7299-9568-0f8eea62e95a" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesCurrent_019ce26e-a34f-79f2-b121-5b9f7e98da3b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_019ce26e-a34f-7128-8961-3efaf06ac90d" xlink:to="loc_us-gaap_LiabilitiesCurrent_019ce26e-a34f-79f2-b121-5b9f7e98da3b" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtAndCapitalLeaseObligations_019ce26e-a34f-7ef6-8599-1bac5727de93" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtAndCapitalLeaseObligations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_019ce26e-a34f-7ece-8509-add4f885ace4" xlink:to="loc_us-gaap_LongTermDebtAndCapitalLeaseObligations_019ce26e-a34f-7ef6-8599-1bac5727de93" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseLiabilityNoncurrent_019ce26e-a34f-77d5-bac3-aba03398b902" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseLiabilityNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_019ce26e-a34f-7ece-8509-add4f885ace4" xlink:to="loc_us-gaap_OperatingLeaseLiabilityNoncurrent_019ce26e-a34f-77d5-bac3-aba03398b902" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccruedCappingClosurePostClosureAndEnvironmentalCostsNoncurrent_019ce26e-a34f-790c-a89e-f07873dde4de" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccruedCappingClosurePostClosureAndEnvironmentalCostsNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_019ce26e-a34f-7ece-8509-add4f885ace4" xlink:to="loc_us-gaap_AccruedCappingClosurePostClosureAndEnvironmentalCostsNoncurrent_019ce26e-a34f-790c-a89e-f07873dde4de" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredIncomeTaxLiabilitiesNet_019ce26e-a34f-7e98-bd5d-3b134efb351b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredIncomeTaxLiabilitiesNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_019ce26e-a34f-7ece-8509-add4f885ace4" xlink:to="loc_us-gaap_DeferredIncomeTaxLiabilitiesNet_019ce26e-a34f-7e98-bd5d-3b134efb351b" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherLiabilitiesNoncurrent_019ce26e-a34f-7fa1-9e58-751320704562" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherLiabilitiesNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_019ce26e-a34f-7ece-8509-add4f885ace4" xlink:to="loc_us-gaap_OtherLiabilitiesNoncurrent_019ce26e-a34f-7fa1-9e58-751320704562" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommitmentsAndContingencies_019ce26e-a34f-7c31-8683-a5a4536d460d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommitmentsAndContingencies"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_019ce26e-a34f-7ece-8509-add4f885ace4" xlink:to="loc_us-gaap_CommitmentsAndContingencies_019ce26e-a34f-7c31-8683-a5a4536d460d" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityAbstract_019ce26e-a34f-7500-b0e8-e959e86dd8d9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquityAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_019ce26e-a34f-7ece-8509-add4f885ace4" xlink:to="loc_us-gaap_StockholdersEquityAbstract_019ce26e-a34f-7500-b0e8-e959e86dd8d9" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockValue_019ce26e-a34f-7e19-8d23-62d587186173" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_019ce26e-a34f-7500-b0e8-e959e86dd8d9" xlink:to="loc_us-gaap_CommonStockValue_019ce26e-a34f-7e19-8d23-62d587186173" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalCommonStock_019ce26e-a34f-7eb2-a6ca-90e69825b8b8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdditionalPaidInCapitalCommonStock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_019ce26e-a34f-7500-b0e8-e959e86dd8d9" xlink:to="loc_us-gaap_AdditionalPaidInCapitalCommonStock_019ce26e-a34f-7eb2-a6ca-90e69825b8b8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsAccumulatedDeficit_019ce26e-a34f-71c2-ab83-a9a9cacedbac" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RetainedEarningsAccumulatedDeficit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_019ce26e-a34f-7500-b0e8-e959e86dd8d9" xlink:to="loc_us-gaap_RetainedEarningsAccumulatedDeficit_019ce26e-a34f-71c2-ab83-a9a9cacedbac" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_019ce26e-a34f-7571-a8c6-8e6a4f10f96e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_019ce26e-a34f-7500-b0e8-e959e86dd8d9" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_019ce26e-a34f-7571-a8c6-8e6a4f10f96e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019ce26e-a34f-7f5d-9e44-f571b5343787" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_019ce26e-a34f-7500-b0e8-e959e86dd8d9" xlink:to="loc_us-gaap_StockholdersEquity_019ce26e-a34f-7f5d-9e44-f571b5343787" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAndStockholdersEquity_019ce26e-a34f-7ad9-91a0-acfb914e6c29" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesAndStockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_019ce26e-a34f-7ece-8509-add4f885ace4" xlink:to="loc_us-gaap_LiabilitiesAndStockholdersEquity_019ce26e-a34f-7ad9-91a0-acfb914e6c29" xlink:type="arc" order="9"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/CONSOLIDATEDBALANCESHEETSParenthetical" xlink:type="simple" xlink:href="cwst-20260331.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_019ce26e-a34f-76aa-b326-fa29eb71ffed" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019ce26e-a34f-7d65-a25e-a010d54897c3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019ce26e-a34f-76aa-b326-fa29eb71ffed" xlink:to="loc_us-gaap_StatementTable_019ce26e-a34f-7d65-a25e-a010d54897c3" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_019ce26e-a34f-7794-b250-e4b483278416" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019ce26e-a34f-7d65-a25e-a010d54897c3" xlink:to="loc_us-gaap_StatementClassOfStockAxis_019ce26e-a34f-7794-b250-e4b483278416" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019ce26e-a34f-7794-b250-e4b483278416_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019ce26e-a34f-7794-b250-e4b483278416" xlink:to="loc_us-gaap_ClassOfStockDomain_019ce26e-a34f-7794-b250-e4b483278416_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019ce26e-a34f-7d94-8a0e-1d8c9a92204b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019ce26e-a34f-7794-b250-e4b483278416" xlink:to="loc_us-gaap_ClassOfStockDomain_019ce26e-a34f-7d94-8a0e-1d8c9a92204b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_019ce26e-a34f-7d59-938a-9125d1bd3efe" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019ce26e-a34f-7d94-8a0e-1d8c9a92204b" xlink:to="loc_us-gaap_CommonClassAMember_019ce26e-a34f-7d59-938a-9125d1bd3efe" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_019ce26e-a34f-7f28-b1f6-179b732af0b3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019ce26e-a34f-7d94-8a0e-1d8c9a92204b" xlink:to="loc_us-gaap_CommonClassBMember_019ce26e-a34f-7f28-b1f6-179b732af0b3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent_019ce26e-a34f-7e85-badb-125ea80c0a4c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019ce26e-a34f-76aa-b326-fa29eb71ffed" xlink:to="loc_us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent_019ce26e-a34f-7e85-badb-125ea80c0a4c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAccumulatedDepreciationAndAmortization_019ce26e-a34f-7162-a2ea-62c2c25008d3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAccumulatedDepreciationAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019ce26e-a34f-76aa-b326-fa29eb71ffed" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAccumulatedDepreciationAndAmortization_019ce26e-a34f-7162-a2ea-62c2c25008d3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_019ce26e-a34f-7e5d-a83a-db3eaf917822" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019ce26e-a34f-76aa-b326-fa29eb71ffed" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_019ce26e-a34f-7e5d-a83a-db3eaf917822" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesAuthorized_019ce26e-a34f-73a5-9289-98a87f66be9f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019ce26e-a34f-76aa-b326-fa29eb71ffed" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_019ce26e-a34f-73a5-9289-98a87f66be9f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesIssued_019ce26e-a34f-75c3-b1a8-02fa2020d6d5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019ce26e-a34f-76aa-b326-fa29eb71ffed" xlink:to="loc_us-gaap_CommonStockSharesIssued_019ce26e-a34f-75c3-b1a8-02fa2020d6d5" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019ce26e-a34f-7f6e-bef3-ab84b8ee0ac8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019ce26e-a34f-76aa-b326-fa29eb71ffed" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_019ce26e-a34f-7f6e-bef3-ab84b8ee0ac8" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_StockholdersEquityNumberOfVotesEachShare_019ce26e-a34f-784a-8be0-8c97f8572cec" xlink:href="cwst-20260331.xsd#cwst_StockholdersEquityNumberOfVotesEachShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019ce26e-a34f-76aa-b326-fa29eb71ffed" xlink:to="loc_cwst_StockholdersEquityNumberOfVotesEachShare_019ce26e-a34f-784a-8be0-8c97f8572cec" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/UNAUDITEDCONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITY" xlink:type="simple" xlink:href="cwst-20260331.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_019ce26e-a34f-7821-bd61-39b712be1f29" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019ce26e-a34f-72df-b15b-ab03d8cf8fe1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019ce26e-a34f-7821-bd61-39b712be1f29" xlink:to="loc_us-gaap_StatementTable_019ce26e-a34f-72df-b15b-ab03d8cf8fe1" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_019ce26e-a34f-787e-91a7-871155de1df7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019ce26e-a34f-72df-b15b-ab03d8cf8fe1" xlink:to="loc_us-gaap_StatementClassOfStockAxis_019ce26e-a34f-787e-91a7-871155de1df7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019ce26e-a34f-787e-91a7-871155de1df7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019ce26e-a34f-787e-91a7-871155de1df7" xlink:to="loc_us-gaap_ClassOfStockDomain_019ce26e-a34f-787e-91a7-871155de1df7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019ce26e-a34f-7c29-b110-ef00dcb61c6e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019ce26e-a34f-787e-91a7-871155de1df7" xlink:to="loc_us-gaap_ClassOfStockDomain_019ce26e-a34f-7c29-b110-ef00dcb61c6e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_019ce26e-a34f-723e-a219-991e34ef1a52" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019ce26e-a34f-7c29-b110-ef00dcb61c6e" xlink:to="loc_us-gaap_CommonClassAMember_019ce26e-a34f-723e-a219-991e34ef1a52" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_019ce26e-a34f-7455-b17d-55f2a46898dc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019ce26e-a34f-7c29-b110-ef00dcb61c6e" xlink:to="loc_us-gaap_CommonClassBMember_019ce26e-a34f-7455-b17d-55f2a46898dc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019ce26e-a34f-76ba-8e69-a8becb388843" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019ce26e-a34f-72df-b15b-ab03d8cf8fe1" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019ce26e-a34f-76ba-8e69-a8becb388843" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019ce26e-a34f-76ba-8e69-a8becb388843_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019ce26e-a34f-76ba-8e69-a8becb388843" xlink:to="loc_us-gaap_EquityComponentDomain_019ce26e-a34f-76ba-8e69-a8becb388843_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019ce26e-a34f-71a1-a70f-18883893b903" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019ce26e-a34f-76ba-8e69-a8becb388843" xlink:to="loc_us-gaap_EquityComponentDomain_019ce26e-a34f-71a1-a70f-18883893b903" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_019ce26e-a34f-7786-891c-cbf2d4509942" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019ce26e-a34f-71a1-a70f-18883893b903" xlink:to="loc_us-gaap_CommonStockMember_019ce26e-a34f-7786-891c-cbf2d4509942" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_019ce26e-a34f-72a9-97e5-56569417ca58" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019ce26e-a34f-71a1-a70f-18883893b903" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_019ce26e-a34f-72a9-97e5-56569417ca58" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_019ce26e-a34f-7bd2-ae97-b9401b625991" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019ce26e-a34f-71a1-a70f-18883893b903" xlink:to="loc_us-gaap_RetainedEarningsMember_019ce26e-a34f-7bd2-ae97-b9401b625991" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019ce26e-a34f-74d9-b42f-77ee163edd4d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019ce26e-a34f-71a1-a70f-18883893b903" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019ce26e-a34f-74d9-b42f-77ee163edd4d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019ce26e-a34f-753f-8c75-1c14c5c3d241" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019ce26e-a34f-7821-bd61-39b712be1f29" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019ce26e-a34f-753f-8c75-1c14c5c3d241" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019ce26e-a34f-75e7-a133-0df88ed6fb46" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019ce26e-a34f-753f-8c75-1c14c5c3d241" xlink:to="loc_us-gaap_StockholdersEquity_019ce26e-a34f-75e7-a133-0df88ed6fb46" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019ce26e-a34f-7175-9f96-5adc6c2d6738" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019ce26e-a34f-753f-8c75-1c14c5c3d241" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_019ce26e-a34f-7175-9f96-5adc6c2d6738" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation_019ce26e-a34f-7cc9-994d-3879b19fda17" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019ce26e-a34f-753f-8c75-1c14c5c3d241" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation_019ce26e-a34f-7cc9-994d-3879b19fda17" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation_019ce26e-a34f-7581-88cb-cf06c6303061" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019ce26e-a34f-753f-8c75-1c14c5c3d241" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation_019ce26e-a34f-7581-88cb-cf06c6303061" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_019ce26e-a34f-7b84-89a5-ebdc13cb05e5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019ce26e-a34f-753f-8c75-1c14c5c3d241" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_019ce26e-a34f-7b84-89a5-ebdc13cb05e5" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ComprehensiveIncomeNetOfTaxAbstract_019d8d4b-4a19-7180-9a32-01560ff61860" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ComprehensiveIncomeNetOfTaxAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019ce26e-a34f-753f-8c75-1c14c5c3d241" xlink:to="loc_us-gaap_ComprehensiveIncomeNetOfTaxAbstract_019d8d4b-4a19-7180-9a32-01560ff61860" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019ce26e-a34f-7600-b4d2-d98dc98d09c2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ComprehensiveIncomeNetOfTaxAbstract_019d8d4b-4a19-7180-9a32-01560ff61860" xlink:to="loc_us-gaap_NetIncomeLoss_019ce26e-a34f-7600-b4d2-d98dc98d09c2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParentAbstract_019d8d4b-b574-74be-a0c3-1e65be998d8b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ComprehensiveIncomeNetOfTaxAbstract_019d8d4b-4a19-7180-9a32-01560ff61860" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParentAbstract_019d8d4b-b574-74be-a0c3-1e65be998d8b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_019ce26e-a34f-721f-a7d7-f311aca37d4e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParentAbstract_019d8d4b-b574-74be-a0c3-1e65be998d8b" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_019ce26e-a34f-721f-a7d7-f311aca37d4e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019ce26e-a34f-702e-bec9-908d330840b4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquity"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019ce26e-a34f-7891-98ce-b0105370d773" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesOutstanding"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/BASISOFPRESENTATIONDetails" xlink:type="simple" xlink:href="cwst-20260331.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_019ce26e-a34f-7438-965f-0196563ff7a2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019ce26e-a34f-76fd-8e52-040ae5e9a32b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ce26e-a34f-7438-965f-0196563ff7a2" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019ce26e-a34f-76fd-8e52-040ae5e9a32b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_019ce26e-a34f-739c-8b1a-f111a63e2fef" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019ce26e-a34f-76fd-8e52-040ae5e9a32b" xlink:to="loc_srt_ProductOrServiceAxis_019ce26e-a34f-739c-8b1a-f111a63e2fef" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019ce26e-a34f-739c-8b1a-f111a63e2fef_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_019ce26e-a34f-739c-8b1a-f111a63e2fef" xlink:to="loc_srt_ProductsAndServicesDomain_019ce26e-a34f-739c-8b1a-f111a63e2fef_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019ce26e-a34f-7c20-a1f2-e703ed046ea6" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_019ce26e-a34f-739c-8b1a-f111a63e2fef" xlink:to="loc_srt_ProductsAndServicesDomain_019ce26e-a34f-7c20-a1f2-e703ed046ea6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_SolidWasteOperationsMember_019ce26e-a34f-7126-bff8-b84e5427f70d" xlink:href="cwst-20260331.xsd#cwst_SolidWasteOperationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019ce26e-a34f-7c20-a1f2-e703ed046ea6" xlink:to="loc_cwst_SolidWasteOperationsMember_019ce26e-a34f-7126-bff8-b84e5427f70d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfStatesInWhichEntityOperates_019ce26e-a34f-7114-bddf-ded691932309" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NumberOfStatesInWhichEntityOperates"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ce26e-a34f-7438-965f-0196563ff7a2" xlink:to="loc_us-gaap_NumberOfStatesInWhichEntityOperates_019ce26e-a34f-7114-bddf-ded691932309" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfOperatingSegments_019ce26e-a34f-7c2f-88b9-37bd146df84a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NumberOfOperatingSegments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ce26e-a34f-7438-965f-0196563ff7a2" xlink:to="loc_us-gaap_NumberOfOperatingSegments_019ce26e-a34f-7c2f-88b9-37bd146df84a" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/REVENUERECOGNITIONScheduleofDisaggregationofRevenueDetails" xlink:type="simple" xlink:href="cwst-20260331.xsd#REVENUERECOGNITIONScheduleofDisaggregationofRevenueDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/REVENUERECOGNITIONScheduleofDisaggregationofRevenueDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_019ce26e-a34f-7366-943b-e07b5068f795" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_019ce26e-a34f-7bc3-8ce2-772280489887" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019ce26e-a34f-7366-943b-e07b5068f795" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_019ce26e-a34f-7bc3-8ce2-772280489887" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019ce26e-a34f-7a09-a7ea-bbf89cc915c0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019ce26e-a34f-7bc3-8ce2-772280489887" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019ce26e-a34f-7a09-a7ea-bbf89cc915c0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019ce26e-a34f-7a09-a7ea-bbf89cc915c0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019ce26e-a34f-7a09-a7ea-bbf89cc915c0" xlink:to="loc_us-gaap_SegmentDomain_019ce26e-a34f-7a09-a7ea-bbf89cc915c0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019ce26e-a34f-7c88-ac7e-2e991fb1979c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019ce26e-a34f-7a09-a7ea-bbf89cc915c0" xlink:to="loc_us-gaap_SegmentDomain_019ce26e-a34f-7c88-ac7e-2e991fb1979c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_EasternRegionMember_019ce26e-a34f-752c-92b3-cd1fa5bfc17f" xlink:href="cwst-20260331.xsd#cwst_EasternRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019ce26e-a34f-7c88-ac7e-2e991fb1979c" xlink:to="loc_cwst_EasternRegionMember_019ce26e-a34f-752c-92b3-cd1fa5bfc17f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_WesternRegionMember_019ce26e-a34f-7127-ac08-ef5706a41947" xlink:href="cwst-20260331.xsd#cwst_WesternRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019ce26e-a34f-7c88-ac7e-2e991fb1979c" xlink:to="loc_cwst_WesternRegionMember_019ce26e-a34f-7127-ac08-ef5706a41947" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_MidAtlanticRegionMember_019ce26e-a34f-7576-bd0c-8db27b4b8cd4" xlink:href="cwst-20260331.xsd#cwst_MidAtlanticRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019ce26e-a34f-7c88-ac7e-2e991fb1979c" xlink:to="loc_cwst_MidAtlanticRegionMember_019ce26e-a34f-7576-bd0c-8db27b4b8cd4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_ResourceSolutionsRegionMember_019ce26e-a34f-7b03-b110-3230b456783b" xlink:href="cwst-20260331.xsd#cwst_ResourceSolutionsRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019ce26e-a34f-7c88-ac7e-2e991fb1979c" xlink:to="loc_cwst_ResourceSolutionsRegionMember_019ce26e-a34f-7b03-b110-3230b456783b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_019ce26e-a34f-78a8-bfda-dffc24070029" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019ce26e-a34f-7bc3-8ce2-772280489887" xlink:to="loc_srt_ProductOrServiceAxis_019ce26e-a34f-78a8-bfda-dffc24070029" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019ce26e-a34f-78a8-bfda-dffc24070029_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_019ce26e-a34f-78a8-bfda-dffc24070029" xlink:to="loc_srt_ProductsAndServicesDomain_019ce26e-a34f-78a8-bfda-dffc24070029_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019ce26e-a34f-776f-a386-08d5e89ff71d" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_019ce26e-a34f-78a8-bfda-dffc24070029" xlink:to="loc_srt_ProductsAndServicesDomain_019ce26e-a34f-776f-a386-08d5e89ff71d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_CollectionMember_019ce26e-a34f-7b0f-836e-a3c8a72f7e60" xlink:href="cwst-20260331.xsd#cwst_CollectionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019ce26e-a34f-776f-a386-08d5e89ff71d" xlink:to="loc_cwst_CollectionMember_019ce26e-a34f-7b0f-836e-a3c8a72f7e60" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_LandfillRevenueMember_019ce26e-a34f-78d2-8f2b-1d01907f5d65" xlink:href="cwst-20260331.xsd#cwst_LandfillRevenueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019ce26e-a34f-776f-a386-08d5e89ff71d" xlink:to="loc_cwst_LandfillRevenueMember_019ce26e-a34f-78d2-8f2b-1d01907f5d65" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_TransferStationMember_019ce26e-a34f-7a10-ab4d-31ce547bd79c" xlink:href="cwst-20260331.xsd#cwst_TransferStationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019ce26e-a34f-776f-a386-08d5e89ff71d" xlink:to="loc_cwst_TransferStationMember_019ce26e-a34f-7a10-ab4d-31ce547bd79c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_TransportationMember_019ce26e-a34f-74d4-bcc3-a45cf5fe00c2" xlink:href="cwst-20260331.xsd#cwst_TransportationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019ce26e-a34f-776f-a386-08d5e89ff71d" xlink:to="loc_cwst_TransportationMember_019ce26e-a34f-74d4-bcc3-a45cf5fe00c2" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_LandfillGasToEnergyMember_019ce26e-a34f-7368-8c22-47074b085ea7" xlink:href="cwst-20260331.xsd#cwst_LandfillGasToEnergyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019ce26e-a34f-776f-a386-08d5e89ff71d" xlink:to="loc_cwst_LandfillGasToEnergyMember_019ce26e-a34f-7368-8c22-47074b085ea7" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_ProcessingServicesMember_019ce26e-a34f-7dc5-9973-250d33f26cef" xlink:href="cwst-20260331.xsd#cwst_ProcessingServicesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019ce26e-a34f-776f-a386-08d5e89ff71d" xlink:to="loc_cwst_ProcessingServicesMember_019ce26e-a34f-7dc5-9973-250d33f26cef" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_NationalAccountsMember_019ce26e-a34f-7302-be7d-cf66e612e89d" xlink:href="cwst-20260331.xsd#cwst_NationalAccountsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019ce26e-a34f-776f-a386-08d5e89ff71d" xlink:to="loc_cwst_NationalAccountsMember_019ce26e-a34f-7302-be7d-cf66e612e89d" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_019ce26e-a34f-7bcd-a54d-f60a5df69a04" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TimingOfTransferOfGoodOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019ce26e-a34f-7bc3-8ce2-772280489887" xlink:to="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_019ce26e-a34f-7bcd-a54d-f60a5df69a04" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_019ce26e-a34f-7bcd-a54d-f60a5df69a04_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TimingOfTransferOfGoodOrServiceDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_019ce26e-a34f-7bcd-a54d-f60a5df69a04" xlink:to="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_019ce26e-a34f-7bcd-a54d-f60a5df69a04_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_019ce26e-a34f-77a2-9c63-ffb413329d17" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TimingOfTransferOfGoodOrServiceDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_019ce26e-a34f-7bcd-a54d-f60a5df69a04" xlink:to="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_019ce26e-a34f-77a2-9c63-ffb413329d17" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransferredAtPointInTimeMember_019ce26e-a34f-7408-8fcc-2853bfc18c26" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransferredAtPointInTimeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_019ce26e-a34f-77a2-9c63-ffb413329d17" xlink:to="loc_us-gaap_TransferredAtPointInTimeMember_019ce26e-a34f-7408-8fcc-2853bfc18c26" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransferredOverTimeMember_019ce26e-a34f-71d1-984a-98c578ad256f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransferredOverTimeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_019ce26e-a34f-77a2-9c63-ffb413329d17" xlink:to="loc_us-gaap_TransferredOverTimeMember_019ce26e-a34f-71d1-984a-98c578ad256f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019ce26e-a34f-75d3-8b04-04b74132b30c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019ce26e-a34f-7366-943b-e07b5068f795" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019ce26e-a34f-75d3-8b04-04b74132b30c" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/REVENUERECOGNITIONNarrativeDetails" xlink:type="simple" xlink:href="cwst-20260331.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_019ce26e-a34f-7f12-a23e-54fe977ba86b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_019ce26e-a34f-7bc8-8a1d-becc590284b6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019ce26e-a34f-7f12-a23e-54fe977ba86b" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_019ce26e-a34f-7bc8-8a1d-becc590284b6" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_019ce26e-a34f-7d74-a9b1-f0da952c7447" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019ce26e-a34f-7bc8-8a1d-becc590284b6" xlink:to="loc_srt_ProductOrServiceAxis_019ce26e-a34f-7d74-a9b1-f0da952c7447" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019ce26e-a34f-7d74-a9b1-f0da952c7447_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_019ce26e-a34f-7d74-a9b1-f0da952c7447" xlink:to="loc_srt_ProductsAndServicesDomain_019ce26e-a34f-7d74-a9b1-f0da952c7447_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019ce26e-a34f-7e5e-9b92-12ff1a96713a" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_019ce26e-a34f-7d74-a9b1-f0da952c7447" xlink:to="loc_srt_ProductsAndServicesDomain_019ce26e-a34f-7e5e-9b92-12ff1a96713a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_RebateRecycledOrReturnedOrganicMaterialsMember_019ce26e-a34f-712f-9863-a1aa11fd3893" xlink:href="cwst-20260331.xsd#cwst_RebateRecycledOrReturnedOrganicMaterialsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019ce26e-a34f-7e5e-9b92-12ff1a96713a" xlink:to="loc_cwst_RebateRecycledOrReturnedOrganicMaterialsMember_019ce26e-a34f-712f-9863-a1aa11fd3893" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019ce26e-a34f-71ab-be74-3b3d50c29fce" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019ce26e-a34f-7f12-a23e-54fe977ba86b" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019ce26e-a34f-71ab-be74-3b3d50c29fce" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerPerformanceObligationSatisfiedInPreviousPeriod_019ce26e-a34f-7728-be07-c4259bcf6a4d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ContractWithCustomerPerformanceObligationSatisfiedInPreviousPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019ce26e-a34f-7f12-a23e-54fe977ba86b" xlink:to="loc_us-gaap_ContractWithCustomerPerformanceObligationSatisfiedInPreviousPeriod_019ce26e-a34f-7728-be07-c4259bcf6a4d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerAssetGrossCurrent_019ce26e-a34f-7f07-bffe-0904266b4d8d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ContractWithCustomerAssetGrossCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019ce26e-a34f-7f12-a23e-54fe977ba86b" xlink:to="loc_us-gaap_ContractWithCustomerAssetGrossCurrent_019ce26e-a34f-7f07-bffe-0904266b4d8d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityCurrent_019ce26e-a34f-7ab0-87e2-6280f2356913" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ContractWithCustomerLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019ce26e-a34f-7f12-a23e-54fe977ba86b" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityCurrent_019ce26e-a34f-7ab0-87e2-6280f2356913" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/BUSINESSCOMBINATIONSNarrativeDetails" xlink:type="simple" xlink:href="cwst-20260331.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_019ce26e-a34f-7ba3-a75d-b7f0667d6f70" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019ce26e-a34f-7642-9c77-d8c239ebbd9b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019ce26e-a34f-7ba3-a75d-b7f0667d6f70" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019ce26e-a34f-7642-9c77-d8c239ebbd9b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019ce26e-a34f-7623-88df-c43db75acc09" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019ce26e-a34f-7642-9c77-d8c239ebbd9b" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019ce26e-a34f-7623-88df-c43db75acc09" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019ce26e-a34f-7623-88df-c43db75acc09_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019ce26e-a34f-7623-88df-c43db75acc09" xlink:to="loc_us-gaap_SegmentDomain_019ce26e-a34f-7623-88df-c43db75acc09_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019ce26e-a34f-7beb-9396-23756b2012cd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019ce26e-a34f-7623-88df-c43db75acc09" xlink:to="loc_us-gaap_SegmentDomain_019ce26e-a34f-7beb-9396-23756b2012cd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_MidAtlanticRegionMember_019ce26e-a34f-7adf-bb0d-0b5a4c11927c" xlink:href="cwst-20260331.xsd#cwst_MidAtlanticRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019ce26e-a34f-7beb-9396-23756b2012cd" xlink:to="loc_cwst_MidAtlanticRegionMember_019ce26e-a34f-7adf-bb0d-0b5a4c11927c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019ce26e-a34f-7dc0-930f-cff9f486bf1f" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019ce26e-a34f-7642-9c77-d8c239ebbd9b" xlink:to="loc_srt_RangeAxis_019ce26e-a34f-7dc0-930f-cff9f486bf1f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019ce26e-a34f-7dc0-930f-cff9f486bf1f_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019ce26e-a34f-7dc0-930f-cff9f486bf1f" xlink:to="loc_srt_RangeMember_019ce26e-a34f-7dc0-930f-cff9f486bf1f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019ce26e-a34f-7122-bf0a-6f94907b9c0b" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019ce26e-a34f-7dc0-930f-cff9f486bf1f" xlink:to="loc_srt_RangeMember_019ce26e-a34f-7122-bf0a-6f94907b9c0b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019ce26e-a34f-7220-b048-468725c6f2d3" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019ce26e-a34f-7122-bf0a-6f94907b9c0b" xlink:to="loc_srt_MinimumMember_019ce26e-a34f-7220-b048-468725c6f2d3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019ce26e-a34f-7b9d-82a4-2011bcfa18ea" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019ce26e-a34f-7122-bf0a-6f94907b9c0b" xlink:to="loc_srt_MaximumMember_019ce26e-a34f-7b9d-82a4-2011bcfa18ea" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_019d747b-e151-7451-8808-8a5842f4ad26" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019ce26e-a34f-7642-9c77-d8c239ebbd9b" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_019d747b-e151-7451-8808-8a5842f4ad26" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_019d747b-e151-7451-8808-8a5842f4ad26_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_019d747b-e151-7451-8808-8a5842f4ad26" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_019d747b-e151-7451-8808-8a5842f4ad26_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_019d747b-e152-71ab-b6d0-a8fefc8b95d1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_019d747b-e151-7451-8808-8a5842f4ad26" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_019d747b-e152-71ab-b6d0-a8fefc8b95d1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventMember_019d747c-1829-7c2c-946e-6ad2d7300df3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_019d747b-e152-71ab-b6d0-a8fefc8b95d1" xlink:to="loc_us-gaap_SubsequentEventMember_019d747c-1829-7c2c-946e-6ad2d7300df3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfBusinessesAcquired_019ce26e-a34f-7757-8285-a4675179e03d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NumberOfBusinessesAcquired"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019ce26e-a34f-7ba3-a75d-b7f0667d6f70" xlink:to="loc_us-gaap_NumberOfBusinessesAcquired_019ce26e-a34f-7757-8285-a4675179e03d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_019ce26e-a34f-7d2c-ba32-2b5edfa9c514" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019ce26e-a34f-7ba3-a75d-b7f0667d6f70" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_019ce26e-a34f-7d2c-ba32-2b5edfa9c514" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/BUSINESSCOMBINATIONSScheduleofPurchasePricePaidforAcquisitionsDetails" xlink:type="simple" xlink:href="cwst-20260331.xsd#BUSINESSCOMBINATIONSScheduleofPurchasePricePaidforAcquisitionsDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/BUSINESSCOMBINATIONSScheduleofPurchasePricePaidforAcquisitionsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_019ce26e-a34f-728e-8af9-c6dbaf937d50" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019ce26e-a34f-796e-959d-094552322220" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019ce26e-a34f-728e-8af9-c6dbaf937d50" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019ce26e-a34f-796e-959d-094552322220" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_019ce26e-a34f-77da-88b4-5ac2d98e2f50" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019ce26e-a34f-796e-959d-094552322220" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_019ce26e-a34f-77da-88b4-5ac2d98e2f50" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019ce26e-a34f-77da-88b4-5ac2d98e2f50_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_019ce26e-a34f-77da-88b4-5ac2d98e2f50" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019ce26e-a34f-77da-88b4-5ac2d98e2f50_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019ce26e-a34f-71ec-a631-ebc0d5935227" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_019ce26e-a34f-77da-88b4-5ac2d98e2f50" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019ce26e-a34f-71ec-a631-ebc0d5935227" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_TransferStationsAcquisitionMember_019ce26e-a34f-71c8-98b0-92a62956bb8e" xlink:href="cwst-20260331.xsd#cwst_TransferStationsAcquisitionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019ce26e-a34f-71ec-a631-ebc0d5935227" xlink:to="loc_cwst_TransferStationsAcquisitionMember_019ce26e-a34f-71c8-98b0-92a62956bb8e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019ce26e-a34f-7b9e-886b-c12e092a12e0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019ce26e-a34f-796e-959d-094552322220" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019ce26e-a34f-7b9e-886b-c12e092a12e0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019ce26e-a34f-7b9e-886b-c12e092a12e0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019ce26e-a34f-7b9e-886b-c12e092a12e0" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019ce26e-a34f-7b9e-886b-c12e092a12e0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019ce26e-a34f-700c-928d-9ffaeae75524" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019ce26e-a34f-7b9e-886b-c12e092a12e0" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019ce26e-a34f-700c-928d-9ffaeae75524" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TradeNamesMember_019ce26e-a34f-7e38-af54-5b274ffe503f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TradeNamesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019ce26e-a34f-700c-928d-9ffaeae75524" xlink:to="loc_us-gaap_TradeNamesMember_019ce26e-a34f-7e38-af54-5b274ffe503f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoncompeteAgreementsMember_019ce26e-a34f-7cf4-b794-514a3499c262" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NoncompeteAgreementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019ce26e-a34f-700c-928d-9ffaeae75524" xlink:to="loc_us-gaap_NoncompeteAgreementsMember_019ce26e-a34f-7cf4-b794-514a3499c262" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_019ce26e-a34f-75ef-af43-a6443388f0f7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CustomerRelationshipsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019ce26e-a34f-700c-928d-9ffaeae75524" xlink:to="loc_us-gaap_CustomerRelationshipsMember_019ce26e-a34f-75ef-af43-a6443388f0f7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferredAbstract_019ce26e-a34f-7368-a311-c7b16ff8bbe0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationConsiderationTransferredAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019ce26e-a34f-728e-8af9-c6dbaf937d50" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferredAbstract_019ce26e-a34f-7368-a311-c7b16ff8bbe0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired_019ce26e-a34f-78d3-96e8-0a7f05b3e7f6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationConsiderationTransferredAbstract_019ce26e-a34f-7368-a311-c7b16ff8bbe0" xlink:to="loc_us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired_019ce26e-a34f-78d3-96e8-0a7f05b3e7f6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAcquiredFromAcquisition_019ce26e-a34f-7221-9205-bcee30820d1a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashAcquiredFromAcquisition"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired_019ce26e-a34f-78d3-96e8-0a7f05b3e7f6" xlink:to="loc_us-gaap_CashAcquiredFromAcquisition_019ce26e-a34f-7221-9205-bcee30820d1a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_BusinessCombinationConsiderationTransferredAdjustmentSettlements_019ce26e-a34f-7940-910a-ceb20fd575a7" xlink:href="cwst-20260331.xsd#cwst_BusinessCombinationConsiderationTransferredAdjustmentSettlements"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationConsiderationTransferredAbstract_019ce26e-a34f-7368-a311-c7b16ff8bbe0" xlink:to="loc_cwst_BusinessCombinationConsiderationTransferredAdjustmentSettlements_019ce26e-a34f-7940-910a-ceb20fd575a7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferred1_019ce26e-a34f-705d-98b0-d3903ac0b025" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationConsiderationTransferred1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationConsiderationTransferredAbstract_019ce26e-a34f-7368-a311-c7b16ff8bbe0" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferred1_019ce26e-a34f-705d-98b0-d3903ac0b025" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNetAbstract_019ce26e-a34f-7122-aa58-15a29cb42fc4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNetAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019ce26e-a34f-728e-8af9-c6dbaf937d50" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNetAbstract_019ce26e-a34f-7122-aa58-15a29cb42fc4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssets_019ce26e-a34f-7f00-84e8-be40ae3acf8d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNetAbstract_019ce26e-a34f-7122-aa58-15a29cb42fc4" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssets_019ce26e-a34f-7f00-84e8-be40ae3acf8d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLand_019ce26e-a34f-7b95-8792-7d21e12ad54b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLand"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNetAbstract_019ce26e-a34f-7122-aa58-15a29cb42fc4" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLand_019ce26e-a34f-7b95-8792-7d21e12ad54b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedBuildings_019ce26e-a34f-7085-b8c5-2d2ee66bcb2a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedBuildings"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNetAbstract_019ce26e-a34f-7122-aa58-15a29cb42fc4" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedBuildings_019ce26e-a34f-7085-b8c5-2d2ee66bcb2a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedEquipment_019ce26e-a34f-7378-9b9a-fb002da35ae4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNetAbstract_019ce26e-a34f-7122-aa58-15a29cb42fc4" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedEquipment_019ce26e-a34f-7378-9b9a-fb002da35ae4" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeaseRightOfUseAsset_019ce26e-a34f-78de-b88c-bde68201456f" xlink:href="cwst-20260331.xsd#cwst_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeaseRightOfUseAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNetAbstract_019ce26e-a34f-7122-aa58-15a29cb42fc4" xlink:to="loc_cwst_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeaseRightOfUseAsset_019ce26e-a34f-78de-b88c-bde68201456f" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill_019ce26e-a34f-79ec-80c2-2265d2fe34ef" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNetAbstract_019ce26e-a34f-7122-aa58-15a29cb42fc4" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill_019ce26e-a34f-79ec-80c2-2265d2fe34ef" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilities_019ce26e-a34f-7ba3-88ee-b56f08919fee" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNetAbstract_019ce26e-a34f-7122-aa58-15a29cb42fc4" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilities_019ce26e-a34f-7ba3-88ee-b56f08919fee" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeaseLiabilityNoncurrent_019ce26e-a34f-756b-a44a-69d0dfa7b2d0" xlink:href="cwst-20260331.xsd#cwst_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeaseLiabilityNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNetAbstract_019ce26e-a34f-7122-aa58-15a29cb42fc4" xlink:to="loc_cwst_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeaseLiabilityNoncurrent_019ce26e-a34f-756b-a44a-69d0dfa7b2d0" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet_019ce26e-a34f-7f0d-8fe6-a6d91fbf4646" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNetAbstract_019ce26e-a34f-7122-aa58-15a29cb42fc4" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet_019ce26e-a34f-7f0d-8fe6-a6d91fbf4646" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_019ce26e-a34f-75cd-afde-e819b40269d1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019ce26e-a34f-728e-8af9-c6dbaf937d50" xlink:to="loc_us-gaap_Goodwill_019ce26e-a34f-75cd-afde-e819b40269d1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables_019ce26e-a34f-7625-a72c-5d0aa7e4d726" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019ce26e-a34f-728e-8af9-c6dbaf937d50" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables_019ce26e-a34f-7625-a72c-5d0aa7e4d726" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/GOODWILLANDINTANGIBLEASSETSScheduleofGoodwillActivityDetails" xlink:type="simple" xlink:href="cwst-20260331.xsd#GOODWILLANDINTANGIBLEASSETSScheduleofGoodwillActivityDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/GOODWILLANDINTANGIBLEASSETSScheduleofGoodwillActivityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillLineItems_019ce26e-a34f-7671-88fb-416ff410ce4a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GoodwillLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfGoodwillTable_019ce26e-a34f-721c-bbb7-563334c93df7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfGoodwillTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_GoodwillLineItems_019ce26e-a34f-7671-88fb-416ff410ce4a" xlink:to="loc_us-gaap_ScheduleOfGoodwillTable_019ce26e-a34f-721c-bbb7-563334c93df7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019ce26e-a34f-77eb-a269-eb6cc85c6ef0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfGoodwillTable_019ce26e-a34f-721c-bbb7-563334c93df7" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019ce26e-a34f-77eb-a269-eb6cc85c6ef0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019ce26e-a34f-77eb-a269-eb6cc85c6ef0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019ce26e-a34f-77eb-a269-eb6cc85c6ef0" xlink:to="loc_us-gaap_SegmentDomain_019ce26e-a34f-77eb-a269-eb6cc85c6ef0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019ce26e-a34f-782d-9f74-c3a8d47f8a3c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019ce26e-a34f-77eb-a269-eb6cc85c6ef0" xlink:to="loc_us-gaap_SegmentDomain_019ce26e-a34f-782d-9f74-c3a8d47f8a3c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_EasternRegionMember_019ce26e-a34f-7c73-be55-ff4da1d178ed" xlink:href="cwst-20260331.xsd#cwst_EasternRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019ce26e-a34f-782d-9f74-c3a8d47f8a3c" xlink:to="loc_cwst_EasternRegionMember_019ce26e-a34f-7c73-be55-ff4da1d178ed" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_WesternRegionMember_019ce26e-a34f-72f9-88de-99eddb7d7e7f" xlink:href="cwst-20260331.xsd#cwst_WesternRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019ce26e-a34f-782d-9f74-c3a8d47f8a3c" xlink:to="loc_cwst_WesternRegionMember_019ce26e-a34f-72f9-88de-99eddb7d7e7f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_MidAtlanticRegionMember_019ce26e-a34f-7232-9284-9c247c712f5b" xlink:href="cwst-20260331.xsd#cwst_MidAtlanticRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019ce26e-a34f-782d-9f74-c3a8d47f8a3c" xlink:to="loc_cwst_MidAtlanticRegionMember_019ce26e-a34f-7232-9284-9c247c712f5b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_ResourceSolutionsRegionMember_019ce26e-a34f-7dcd-baba-6150349071d6" xlink:href="cwst-20260331.xsd#cwst_ResourceSolutionsRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019ce26e-a34f-782d-9f74-c3a8d47f8a3c" xlink:to="loc_cwst_ResourceSolutionsRegionMember_019ce26e-a34f-7dcd-baba-6150349071d6" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillRollForward_019ce26e-a34f-7d35-b55d-d4e9055a2dc0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GoodwillRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillLineItems_019ce26e-a34f-7671-88fb-416ff410ce4a" xlink:to="loc_us-gaap_GoodwillRollForward_019ce26e-a34f-7d35-b55d-d4e9055a2dc0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_019ce26e-a34f-7683-a399-d9908ff4e669" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillRollForward_019ce26e-a34f-7d35-b55d-d4e9055a2dc0" xlink:to="loc_us-gaap_Goodwill_019ce26e-a34f-7683-a399-d9908ff4e669" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillAcquiredDuringPeriod_019ce26e-a34f-7a96-b696-f1b61124d1a4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GoodwillAcquiredDuringPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillRollForward_019ce26e-a34f-7d35-b55d-d4e9055a2dc0" xlink:to="loc_us-gaap_GoodwillAcquiredDuringPeriod_019ce26e-a34f-7a96-b696-f1b61124d1a4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillPurchaseAccountingAdjustments_019ce26e-a34f-7c6e-9173-ca26bb8638a9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GoodwillPurchaseAccountingAdjustments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillRollForward_019ce26e-a34f-7d35-b55d-d4e9055a2dc0" xlink:to="loc_us-gaap_GoodwillPurchaseAccountingAdjustments_019ce26e-a34f-7c6e-9173-ca26bb8638a9" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_019ce26e-a34f-7b09-97ee-aa854337e06f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Goodwill"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/GOODWILLANDINTANGIBLEASSETSScheduleofIntangibleAssetsbyTypeDetails" xlink:type="simple" xlink:href="cwst-20260331.xsd#GOODWILLANDINTANGIBLEASSETSScheduleofIntangibleAssetsbyTypeDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/GOODWILLANDINTANGIBLEASSETSScheduleofIntangibleAssetsbyTypeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019ce26e-a34f-707b-9c55-aa7f3362247b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_019ce26e-a34f-7869-8e4d-aeef018c6cf2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019ce26e-a34f-707b-9c55-aa7f3362247b" xlink:to="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_019ce26e-a34f-7869-8e4d-aeef018c6cf2" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019ce26e-a34f-784c-b37e-63f812f2b828" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_019ce26e-a34f-7869-8e4d-aeef018c6cf2" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019ce26e-a34f-784c-b37e-63f812f2b828" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019ce26e-a34f-784c-b37e-63f812f2b828_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019ce26e-a34f-784c-b37e-63f812f2b828" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019ce26e-a34f-784c-b37e-63f812f2b828_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019ce26e-a34f-7d09-8026-9612cee3931d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019ce26e-a34f-784c-b37e-63f812f2b828" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019ce26e-a34f-7d09-8026-9612cee3931d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoncompeteAgreementsMember_019ce26e-a34f-7a46-802a-bfe69ef10383" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NoncompeteAgreementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019ce26e-a34f-7d09-8026-9612cee3931d" xlink:to="loc_us-gaap_NoncompeteAgreementsMember_019ce26e-a34f-7a46-802a-bfe69ef10383" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_019ce26e-a34f-76d9-9f6b-aa89b241ffdc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CustomerRelationshipsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019ce26e-a34f-7d09-8026-9612cee3931d" xlink:to="loc_us-gaap_CustomerRelationshipsMember_019ce26e-a34f-76d9-9f6b-aa89b241ffdc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TradeNamesMember_019ce26e-a34f-7bc2-87b9-4374ede0a3b5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TradeNamesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019ce26e-a34f-7d09-8026-9612cee3931d" xlink:to="loc_us-gaap_TradeNamesMember_019ce26e-a34f-7bc2-87b9-4374ede0a3b5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsGross_019ce26e-a34f-7f3a-8f04-0c215538f5ab" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019ce26e-a34f-707b-9c55-aa7f3362247b" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsGross_019ce26e-a34f-7f3a-8f04-0c215538f5ab" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_019ce26e-a34f-7f91-bb51-58bed67ee90a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019ce26e-a34f-707b-9c55-aa7f3362247b" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_019ce26e-a34f-7f91-bb51-58bed67ee90a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsNet_019ce26e-a34f-7e0a-8766-8e9ee37bc729" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019ce26e-a34f-707b-9c55-aa7f3362247b" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsNet_019ce26e-a34f-7e0a-8766-8e9ee37bc729" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/DEBTScheduleofDebtDetails" xlink:type="simple" xlink:href="cwst-20260331.xsd#DEBTScheduleofDebtDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/DEBTScheduleofDebtDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019ce26e-a34f-7603-b27b-6b59590ec578" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019ce26e-a34f-77be-a47a-92f918c4732f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ce26e-a34f-7603-b27b-6b59590ec578" xlink:to="loc_us-gaap_DebtInstrumentTable_019ce26e-a34f-77be-a47a-92f918c4732f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019ce26e-a34f-7a77-beee-86a3521218d1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019ce26e-a34f-77be-a47a-92f918c4732f" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019ce26e-a34f-7a77-beee-86a3521218d1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019ce26e-a34f-7a77-beee-86a3521218d1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019ce26e-a34f-7a77-beee-86a3521218d1" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019ce26e-a34f-7a77-beee-86a3521218d1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019ce26e-a34f-75d6-b017-fdbcb54b22c1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019ce26e-a34f-7a77-beee-86a3521218d1" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019ce26e-a34f-75d6-b017-fdbcb54b22c1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_019ce26e-a34f-70e6-8a64-c83a0b9ac9c1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019ce26e-a34f-75d6-b017-fdbcb54b22c1" xlink:to="loc_us-gaap_SecuredDebtMember_019ce26e-a34f-70e6-8a64-c83a0b9ac9c1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_019ce26e-a34f-7bf1-82c9-671e920e1675" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019ce26e-a34f-75d6-b017-fdbcb54b22c1" xlink:to="loc_us-gaap_LineOfCreditMember_019ce26e-a34f-7bf1-82c9-671e920e1675" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnsecuredDebtMember_019ce26e-a34f-792a-9191-961c7758a85b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_UnsecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019ce26e-a34f-75d6-b017-fdbcb54b22c1" xlink:to="loc_us-gaap_UnsecuredDebtMember_019ce26e-a34f-792a-9191-961c7758a85b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_NotesPayableMember_019ce26e-a34f-7cc3-8b30-d38c945034c6" xlink:href="cwst-20260331.xsd#cwst_NotesPayableMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019ce26e-a34f-75d6-b017-fdbcb54b22c1" xlink:to="loc_cwst_NotesPayableMember_019ce26e-a34f-7cc3-8b30-d38c945034c6" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019ce26e-a34f-7a13-bdb7-00c4d4e37a31" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019ce26e-a34f-77be-a47a-92f918c4732f" xlink:to="loc_us-gaap_DebtInstrumentAxis_019ce26e-a34f-7a13-bdb7-00c4d4e37a31" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019ce26e-a34f-7a13-bdb7-00c4d4e37a31_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019ce26e-a34f-7a13-bdb7-00c4d4e37a31" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019ce26e-a34f-7a13-bdb7-00c4d4e37a31_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019ce26e-a34f-7b05-97a2-4f817db8161b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019ce26e-a34f-7a13-bdb7-00c4d4e37a31" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019ce26e-a34f-7b05-97a2-4f817db8161b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_TermLoanAFacilityDueSeptember2029Member_019ce26e-a34f-765a-b7d7-30fd049dc35d" xlink:href="cwst-20260331.xsd#cwst_TermLoanAFacilityDueSeptember2029Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019ce26e-a34f-7b05-97a2-4f817db8161b" xlink:to="loc_cwst_TermLoanAFacilityDueSeptember2029Member_019ce26e-a34f-765a-b7d7-30fd049dc35d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_RevolvingCreditFacilityDueSeptember2029Member_019ce26e-a34f-7a92-ad1d-838e437b65a3" xlink:href="cwst-20260331.xsd#cwst_RevolvingCreditFacilityDueSeptember2029Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019ce26e-a34f-7b05-97a2-4f817db8161b" xlink:to="loc_cwst_RevolvingCreditFacilityDueSeptember2029Member_019ce26e-a34f-7a92-ad1d-838e437b65a3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_RevenueBondsSeriesTwoThousandFourteenDueTwoThousandFortyFourMember_019ce26e-a34f-7486-b17a-ba3e7bf97f5b" xlink:href="cwst-20260331.xsd#cwst_RevenueBondsSeriesTwoThousandFourteenDueTwoThousandFortyFourMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019ce26e-a34f-7b05-97a2-4f817db8161b" xlink:to="loc_cwst_RevenueBondsSeriesTwoThousandFourteenDueTwoThousandFortyFourMember_019ce26e-a34f-7486-b17a-ba3e7bf97f5b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_RevenueBondSeriesTwoThousandFourteenRTwoDueTwoThousandFortyFourMember_019ce26e-a34f-7a4a-99f0-906e64ef0777" xlink:href="cwst-20260331.xsd#cwst_RevenueBondSeriesTwoThousandFourteenRTwoDueTwoThousandFortyFourMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019ce26e-a34f-7b05-97a2-4f817db8161b" xlink:to="loc_cwst_RevenueBondSeriesTwoThousandFourteenRTwoDueTwoThousandFortyFourMember_019ce26e-a34f-7a4a-99f0-906e64ef0777" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_RevenueBondsSeries2020DueSeptember2050Member_019ce26e-a34f-74f2-aad5-f0b695a36fb1" xlink:href="cwst-20260331.xsd#cwst_RevenueBondsSeries2020DueSeptember2050Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019ce26e-a34f-7b05-97a2-4f817db8161b" xlink:to="loc_cwst_RevenueBondsSeries2020DueSeptember2050Member_019ce26e-a34f-74f2-aad5-f0b695a36fb1" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_RevenueBondsSeries2020R2DueSeptember2050Member_019ce26e-a34f-7fb7-8755-bce4677c9d3a" xlink:href="cwst-20260331.xsd#cwst_RevenueBondsSeries2020R2DueSeptember2050Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019ce26e-a34f-7b05-97a2-4f817db8161b" xlink:to="loc_cwst_RevenueBondsSeries2020R2DueSeptember2050Member_019ce26e-a34f-7fb7-8755-bce4677c9d3a" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_FinanceAuthorityOfMaineSolidWasteDisposalRevenueBondsSeries2015R3Member_019ce26e-a34f-765c-82c6-87aaa6431b37" xlink:href="cwst-20260331.xsd#cwst_FinanceAuthorityOfMaineSolidWasteDisposalRevenueBondsSeries2015R3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019ce26e-a34f-7b05-97a2-4f817db8161b" xlink:to="loc_cwst_FinanceAuthorityOfMaineSolidWasteDisposalRevenueBondsSeries2015R3Member_019ce26e-a34f-765c-82c6-87aaa6431b37" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_FinanceAuthorityOfMaineSolidWasteDisposalRevenueBondsSeries2024Member_019ce26e-a34f-7e09-856d-5742bd791aa3" xlink:href="cwst-20260331.xsd#cwst_FinanceAuthorityOfMaineSolidWasteDisposalRevenueBondsSeries2024Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019ce26e-a34f-7b05-97a2-4f817db8161b" xlink:to="loc_cwst_FinanceAuthorityOfMaineSolidWasteDisposalRevenueBondsSeries2024Member_019ce26e-a34f-7e09-856d-5742bd791aa3" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_LongTermRevenueBondSeriesTwoThousandThirteenDueTwoThousandThirtySixMember_019ce26e-a34f-7413-89e9-47d40556d9c6" xlink:href="cwst-20260331.xsd#cwst_LongTermRevenueBondSeriesTwoThousandThirteenDueTwoThousandThirtySixMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019ce26e-a34f-7b05-97a2-4f817db8161b" xlink:to="loc_cwst_LongTermRevenueBondSeriesTwoThousandThirteenDueTwoThousandThirtySixMember_019ce26e-a34f-7413-89e9-47d40556d9c6" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_VermontBondsSeries2022A1Due2052Member_019ce26e-a34f-7fc6-a34d-f7f5a92b4844" xlink:href="cwst-20260331.xsd#cwst_VermontBondsSeries2022A1Due2052Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019ce26e-a34f-7b05-97a2-4f817db8161b" xlink:to="loc_cwst_VermontBondsSeries2022A1Due2052Member_019ce26e-a34f-7fc6-a34d-f7f5a92b4844" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_VermontBondsSeries2022A2Due2052Member_019ce26e-a34f-7bb6-9452-e6eca8ea3e4a" xlink:href="cwst-20260331.xsd#cwst_VermontBondsSeries2022A2Due2052Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019ce26e-a34f-7b05-97a2-4f817db8161b" xlink:to="loc_cwst_VermontBondsSeries2022A2Due2052Member_019ce26e-a34f-7bb6-9452-e6eca8ea3e4a" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_RevenueBondSeriesTwoThousandThirteenDueTwoThousandTwentyNineMember_019ce26e-a34f-7196-b908-d9593b9785fd" xlink:href="cwst-20260331.xsd#cwst_RevenueBondSeriesTwoThousandThirteenDueTwoThousandTwentyNineMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019ce26e-a34f-7b05-97a2-4f817db8161b" xlink:to="loc_cwst_RevenueBondSeriesTwoThousandThirteenDueTwoThousandTwentyNineMember_019ce26e-a34f-7196-b908-d9593b9785fd" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019ce26e-a34f-72e1-a763-9c78049f41a3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ce26e-a34f-7603-b27b-6b59590ec578" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019ce26e-a34f-72e1-a763-9c78049f41a3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_019ce26e-a34f-7318-b507-03b719902bb2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ce26e-a34f-7603-b27b-6b59590ec578" xlink:to="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_019ce26e-a34f-7318-b507-03b719902bb2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinanceLeaseWeightedAverageDiscountRatePercent_019ce26e-a34f-7454-b046-dac8b13a6807" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinanceLeaseWeightedAverageDiscountRatePercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ce26e-a34f-7603-b27b-6b59590ec578" xlink:to="loc_us-gaap_FinanceLeaseWeightedAverageDiscountRatePercent_019ce26e-a34f-7454-b046-dac8b13a6807" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentCarryingAmount_019ce26e-a34f-7073-b105-17a0382c24d3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ce26e-a34f-7603-b27b-6b59590ec578" xlink:to="loc_us-gaap_DebtInstrumentCarryingAmount_019ce26e-a34f-7073-b105-17a0382c24d3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinanceLeaseLiability_019ce26e-a34f-7e56-864b-32b2b28ff18a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinanceLeaseLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ce26e-a34f-7603-b27b-6b59590ec578" xlink:to="loc_us-gaap_FinanceLeaseLiability_019ce26e-a34f-7e56-864b-32b2b28ff18a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinanceLeaseLiabilityStatementOfFinancialPositionExtensibleList_019ce26e-a34f-7e7d-8632-6b08c92d02c2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinanceLeaseLiabilityStatementOfFinancialPositionExtensibleList"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinanceLeaseLiability_019ce26e-a34f-7e56-864b-32b2b28ff18a" xlink:to="loc_us-gaap_FinanceLeaseLiabilityStatementOfFinancialPositionExtensibleList_019ce26e-a34f-7e7d-8632-6b08c92d02c2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtAndCapitalLeaseObligations_019ce26e-a34f-7905-abd2-547cefa6b97c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtAndCapitalLeaseObligations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ce26e-a34f-7603-b27b-6b59590ec578" xlink:to="loc_us-gaap_DebtAndCapitalLeaseObligations_019ce26e-a34f-7905-abd2-547cefa6b97c" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredFinanceCostsNet_019ce26e-a34f-7ed7-91b9-91e3f4ae1159" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredFinanceCostsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ce26e-a34f-7603-b27b-6b59590ec578" xlink:to="loc_us-gaap_DeferredFinanceCostsNet_019ce26e-a34f-7ed7-91b9-91e3f4ae1159" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtAndCapitalLeaseObligationsIncludingCurrentMaturities_019ce26e-a34f-73f4-866b-c83b7fd349e0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtAndCapitalLeaseObligationsIncludingCurrentMaturities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ce26e-a34f-7603-b27b-6b59590ec578" xlink:to="loc_us-gaap_LongTermDebtAndCapitalLeaseObligationsIncludingCurrentMaturities_019ce26e-a34f-73f4-866b-c83b7fd349e0" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtAndCapitalLeaseObligationsCurrent_019ce26e-a34f-77bf-b33f-118378eca52e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtAndCapitalLeaseObligationsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ce26e-a34f-7603-b27b-6b59590ec578" xlink:to="loc_us-gaap_LongTermDebtAndCapitalLeaseObligationsCurrent_019ce26e-a34f-77bf-b33f-118378eca52e" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtAndCapitalLeaseObligations_019ce26e-a34f-7ac9-a628-5160a14e3541" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtAndCapitalLeaseObligations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ce26e-a34f-7603-b27b-6b59590ec578" xlink:to="loc_us-gaap_LongTermDebtAndCapitalLeaseObligations_019ce26e-a34f-7ac9-a628-5160a14e3541" xlink:type="arc" order="9"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/DEBTNarrativeCreditFacilityDetails" xlink:type="simple" xlink:href="cwst-20260331.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_019cff57-5124-7e7c-9450-8b649c12e5c6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditFacilityLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityTable_019ce26e-a34f-76f7-bd2b-62bb950b8dec" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditFacilityTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019cff57-5124-7e7c-9450-8b649c12e5c6" xlink:to="loc_us-gaap_LineOfCreditFacilityTable_019ce26e-a34f-76f7-bd2b-62bb950b8dec" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019cff57-5124-755d-b33f-ab0ad292c251" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_019ce26e-a34f-76f7-bd2b-62bb950b8dec" xlink:to="loc_us-gaap_DebtInstrumentAxis_019cff57-5124-755d-b33f-ab0ad292c251" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019cff57-5124-755d-b33f-ab0ad292c251_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019cff57-5124-755d-b33f-ab0ad292c251" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019cff57-5124-755d-b33f-ab0ad292c251_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019cff57-5124-7ae6-8015-499f9428086b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019cff57-5124-755d-b33f-ab0ad292c251" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019cff57-5124-7ae6-8015-499f9428086b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_CreditFacilityMember_019cff57-5124-7016-a134-a4bc1b79e504" xlink:href="cwst-20260331.xsd#cwst_CreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019cff57-5124-7ae6-8015-499f9428086b" xlink:to="loc_cwst_CreditFacilityMember_019cff57-5124-7016-a134-a4bc1b79e504" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019cff57-5124-7b6d-9bbc-61ac09d3da37" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_019ce26e-a34f-76f7-bd2b-62bb950b8dec" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019cff57-5124-7b6d-9bbc-61ac09d3da37" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019cff57-5124-7b6d-9bbc-61ac09d3da37_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019cff57-5124-7b6d-9bbc-61ac09d3da37" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019cff57-5124-7b6d-9bbc-61ac09d3da37_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019cff57-5124-79b1-9a26-7679167a00f2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019cff57-5124-7b6d-9bbc-61ac09d3da37" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019cff57-5124-79b1-9a26-7679167a00f2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_019cff57-5124-790c-9aaa-c0d03c1ceb48" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019cff57-5124-79b1-9a26-7679167a00f2" xlink:to="loc_us-gaap_SecuredDebtMember_019cff57-5124-790c-9aaa-c0d03c1ceb48" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_019cff57-5124-778e-ad61-15aa3fb5a1c6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019cff57-5124-79b1-9a26-7679167a00f2" xlink:to="loc_us-gaap_LineOfCreditMember_019cff57-5124-778e-ad61-15aa3fb5a1c6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_019cff57-5124-7853-83ad-1a9db66bd811" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_019ce26e-a34f-76f7-bd2b-62bb950b8dec" xlink:to="loc_us-gaap_CreditFacilityAxis_019cff57-5124-7853-83ad-1a9db66bd811" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019cff57-5124-7853-83ad-1a9db66bd811_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_019cff57-5124-7853-83ad-1a9db66bd811" xlink:to="loc_us-gaap_CreditFacilityDomain_019cff57-5124-7853-83ad-1a9db66bd811_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019cff57-5124-75c4-bfcc-e8d3ff621e79" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_019cff57-5124-7853-83ad-1a9db66bd811" xlink:to="loc_us-gaap_CreditFacilityDomain_019cff57-5124-75c4-bfcc-e8d3ff621e79" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_019cff57-5124-7164-b7f7-e60c7a0cf45f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_019cff57-5124-75c4-bfcc-e8d3ff621e79" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_019cff57-5124-7164-b7f7-e60c7a0cf45f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LetterOfCreditMember_019cff57-7368-709c-a26c-d6108ffd6f6c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LetterOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_019cff57-5124-75c4-bfcc-e8d3ff621e79" xlink:to="loc_us-gaap_LetterOfCreditMember_019cff57-7368-709c-a26c-d6108ffd6f6c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019cff57-7368-755c-8b50-3001cf92ee2a" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_019ce26e-a34f-76f7-bd2b-62bb950b8dec" xlink:to="loc_srt_RangeAxis_019cff57-7368-755c-8b50-3001cf92ee2a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019cff57-7368-755c-8b50-3001cf92ee2a_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019cff57-7368-755c-8b50-3001cf92ee2a" xlink:to="loc_srt_RangeMember_019cff57-7368-755c-8b50-3001cf92ee2a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019cff57-7368-7081-b998-f70f8808d1df" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019cff57-7368-755c-8b50-3001cf92ee2a" xlink:to="loc_srt_RangeMember_019cff57-7368-7081-b998-f70f8808d1df" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019cff57-7368-7387-92da-028e4185c8fe" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019cff57-7368-7081-b998-f70f8808d1df" xlink:to="loc_srt_MinimumMember_019cff57-7368-7387-92da-028e4185c8fe" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019cff57-7368-7094-8161-3c432a7cd6dc" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019cff57-7368-7081-b998-f70f8808d1df" xlink:to="loc_srt_MaximumMember_019cff57-7368-7094-8161-3c432a7cd6dc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_019cff57-5124-7671-b48d-009715e064e6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019cff57-5124-7e7c-9450-8b649c12e5c6" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_019cff57-5124-7671-b48d-009715e064e6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_019cff57-5124-7c5b-8865-cebd57c49808" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019cff57-5124-7e7c-9450-8b649c12e5c6" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_019cff57-5124-7c5b-8865-cebd57c49808" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_LettersOfCreditSublimit_019cff57-5124-7533-a835-0f55d8fa9a75" xlink:href="cwst-20260331.xsd#cwst_LettersOfCreditSublimit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019cff57-5124-7e7c-9450-8b649c12e5c6" xlink:to="loc_cwst_LettersOfCreditSublimit_019cff57-5124-7533-a835-0f55d8fa9a75" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_LineOfCreditFacilityAdditionalBorrowingCapacity_019cff57-5124-7f93-9073-6994d9e55fa0" xlink:href="cwst-20260331.xsd#cwst_LineOfCreditFacilityAdditionalBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019cff57-5124-7e7c-9450-8b649c12e5c6" xlink:to="loc_cwst_LineOfCreditFacilityAdditionalBorrowingCapacity_019cff57-5124-7f93-9073-6994d9e55fa0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTerm_019cff57-5124-766e-9184-97306b37fe51" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019cff57-5124-7e7c-9450-8b649c12e5c6" xlink:to="loc_us-gaap_DebtInstrumentTerm_019cff57-5124-766e-9184-97306b37fe51" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_LineOfCreditFacilityCommitmentFeeBasisSpread_019cff57-7368-7143-bebe-4c6ace095c05" xlink:href="cwst-20260331.xsd#cwst_LineOfCreditFacilityCommitmentFeeBasisSpread"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019cff57-5124-7e7c-9450-8b649c12e5c6" xlink:to="loc_cwst_LineOfCreditFacilityCommitmentFeeBasisSpread_019cff57-7368-7143-bebe-4c6ace095c05" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_LineOfCreditFacilityCommitmentFeePercentageAdjustment_019cff57-7368-7a71-b36d-fde1a5005b0d" xlink:href="cwst-20260331.xsd#cwst_LineOfCreditFacilityCommitmentFeePercentageAdjustment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019cff57-5124-7e7c-9450-8b649c12e5c6" xlink:to="loc_cwst_LineOfCreditFacilityCommitmentFeePercentageAdjustment_019cff57-7368-7a71-b36d-fde1a5005b0d" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_DebtInstrumentFloorInterestRate_019cff57-7368-77bd-b8dc-d75a1849ba56" xlink:href="cwst-20260331.xsd#cwst_DebtInstrumentFloorInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019cff57-5124-7e7c-9450-8b649c12e5c6" xlink:to="loc_cwst_DebtInstrumentFloorInterestRate_019cff57-7368-77bd-b8dc-d75a1849ba56" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_LineOfCreditFacilityFrontingFeePercentage_019cff57-7368-78c9-96a7-c6e8d8943978" xlink:href="cwst-20260331.xsd#cwst_LineOfCreditFacilityFrontingFeePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019cff57-5124-7e7c-9450-8b649c12e5c6" xlink:to="loc_cwst_LineOfCreditFacilityFrontingFeePercentage_019cff57-7368-78c9-96a7-c6e8d8943978" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_DebtInstrumentInterestRateAnnualIncreaseUponPaymentDefault_019cff57-7368-7002-a1d1-3f0760be9072" xlink:href="cwst-20260331.xsd#cwst_DebtInstrumentInterestRateAnnualIncreaseUponPaymentDefault"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019cff57-5124-7e7c-9450-8b649c12e5c6" xlink:to="loc_cwst_DebtInstrumentInterestRateAnnualIncreaseUponPaymentDefault_019cff57-7368-7002-a1d1-3f0760be9072" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_DebtInstrumentInterestRateAnnualIncreaseUponOtherEventOfDefault_019cff57-7368-732a-99e2-47bd0bac4910" xlink:href="cwst-20260331.xsd#cwst_DebtInstrumentInterestRateAnnualIncreaseUponOtherEventOfDefault"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019cff57-5124-7e7c-9450-8b649c12e5c6" xlink:to="loc_cwst_DebtInstrumentInterestRateAnnualIncreaseUponOtherEventOfDefault_019cff57-7368-732a-99e2-47bd0bac4910" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity_019cff57-7368-7176-91b5-950fde000713" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019cff57-5124-7e7c-9450-8b649c12e5c6" xlink:to="loc_us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity_019cff57-7368-7176-91b5-950fde000713" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LettersOfCreditOutstandingAmount_019cff57-7368-7caa-ba7d-aef265d36418" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LettersOfCreditOutstandingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019cff57-5124-7e7c-9450-8b649c12e5c6" xlink:to="loc_us-gaap_LettersOfCreditOutstandingAmount_019cff57-7368-7caa-ba7d-aef265d36418" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCredit_019cff57-7368-7eb7-890d-532a78096581" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCredit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019cff57-5124-7e7c-9450-8b649c12e5c6" xlink:to="loc_us-gaap_LineOfCredit_019cff57-7368-7eb7-890d-532a78096581" xlink:type="arc" order="13"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/DEBTScheduleofVariableRateMarginsDetails" xlink:type="simple" xlink:href="cwst-20260331.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_019ce26e-a34f-744e-ae21-bfb5db870cf1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019ce26e-a34f-7f19-b335-85e6b3d334e0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ce26e-a34f-744e-ae21-bfb5db870cf1" xlink:to="loc_us-gaap_DebtInstrumentTable_019ce26e-a34f-7f19-b335-85e6b3d334e0" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019ce26e-a34f-79ff-b7c1-452c34393497" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019ce26e-a34f-7f19-b335-85e6b3d334e0" xlink:to="loc_us-gaap_DebtInstrumentAxis_019ce26e-a34f-79ff-b7c1-452c34393497" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019ce26e-a34f-79ff-b7c1-452c34393497_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019ce26e-a34f-79ff-b7c1-452c34393497" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019ce26e-a34f-79ff-b7c1-452c34393497_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019ce26e-a34f-77ac-a8c3-d21487c6be86" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019ce26e-a34f-79ff-b7c1-452c34393497" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019ce26e-a34f-77ac-a8c3-d21487c6be86" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_CreditFacilityMember_019ce26e-a34f-7be4-9290-68ad115fc30d" xlink:href="cwst-20260331.xsd#cwst_CreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019ce26e-a34f-77ac-a8c3-d21487c6be86" xlink:to="loc_cwst_CreditFacilityMember_019ce26e-a34f-7be4-9290-68ad115fc30d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019ce26e-a34f-7d3b-9d35-bda6896c3930" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019ce26e-a34f-7f19-b335-85e6b3d334e0" xlink:to="loc_srt_RangeAxis_019ce26e-a34f-7d3b-9d35-bda6896c3930" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019ce26e-a34f-7d3b-9d35-bda6896c3930_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019ce26e-a34f-7d3b-9d35-bda6896c3930" xlink:to="loc_srt_RangeMember_019ce26e-a34f-7d3b-9d35-bda6896c3930_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019ce26e-a34f-70d0-addd-ce1ee73997e9" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019ce26e-a34f-7d3b-9d35-bda6896c3930" xlink:to="loc_srt_RangeMember_019ce26e-a34f-70d0-addd-ce1ee73997e9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019ce26e-a34f-7d01-84c0-b40e05b88c94" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019ce26e-a34f-70d0-addd-ce1ee73997e9" xlink:to="loc_srt_MinimumMember_019ce26e-a34f-7d01-84c0-b40e05b88c94" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019ce26e-a34f-78f7-9101-1be06cb13a23" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019ce26e-a34f-70d0-addd-ce1ee73997e9" xlink:to="loc_srt_MaximumMember_019ce26e-a34f-78f7-9101-1be06cb13a23" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019ce26e-a34f-7753-92e5-ec36de1e141b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019ce26e-a34f-7f19-b335-85e6b3d334e0" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019ce26e-a34f-7753-92e5-ec36de1e141b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019ce26e-a34f-7753-92e5-ec36de1e141b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019ce26e-a34f-7753-92e5-ec36de1e141b" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019ce26e-a34f-7753-92e5-ec36de1e141b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019ce26e-a34f-785f-8a36-0e920df6d837" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019ce26e-a34f-7753-92e5-ec36de1e141b" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019ce26e-a34f-785f-8a36-0e920df6d837" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_019ce26e-a34f-7426-b192-5b990aa70b00" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019ce26e-a34f-785f-8a36-0e920df6d837" xlink:to="loc_us-gaap_LineOfCreditMember_019ce26e-a34f-7426-b192-5b990aa70b00" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateAxis_019ce26e-a34f-7c08-af8f-e298967d2bfc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VariableRateAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019ce26e-a34f-7f19-b335-85e6b3d334e0" xlink:to="loc_us-gaap_VariableRateAxis_019ce26e-a34f-7c08-af8f-e298967d2bfc" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_019ce26e-a34f-7c08-af8f-e298967d2bfc_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VariableRateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VariableRateAxis_019ce26e-a34f-7c08-af8f-e298967d2bfc" xlink:to="loc_us-gaap_VariableRateDomain_019ce26e-a34f-7c08-af8f-e298967d2bfc_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_019ce26e-a34f-7d3f-b089-3d882e007d39" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VariableRateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VariableRateAxis_019ce26e-a34f-7c08-af8f-e298967d2bfc" xlink:to="loc_us-gaap_VariableRateDomain_019ce26e-a34f-7d3f-b089-3d882e007d39" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredOvernightFinancingRateSofrMember_019ce26e-a34f-7b4f-b70b-7908c810685d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SecuredOvernightFinancingRateSofrMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_019ce26e-a34f-7d3f-b089-3d882e007d39" xlink:to="loc_us-gaap_SecuredOvernightFinancingRateSofrMember_019ce26e-a34f-7b4f-b70b-7908c810685d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BaseRateMember_019ce26e-a34f-74d6-9abf-becb3fdae417" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BaseRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_019ce26e-a34f-7d3f-b089-3d882e007d39" xlink:to="loc_us-gaap_BaseRateMember_019ce26e-a34f-74d6-9abf-becb3fdae417" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_019ce26e-a34f-7996-8e34-e7901f687571" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ce26e-a34f-744e-ae21-bfb5db870cf1" xlink:to="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_019ce26e-a34f-7996-8e34-e7901f687571" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_DebtInstrumentVariableRateAdjustment_019cff5b-22ff-7232-9bc9-74e5d436f801" xlink:href="cwst-20260331.xsd#cwst_DebtInstrumentVariableRateAdjustment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ce26e-a34f-744e-ae21-bfb5db870cf1" xlink:to="loc_cwst_DebtInstrumentVariableRateAdjustment_019cff5b-22ff-7232-9bc9-74e5d436f801" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/DEBTNarrativeTaxExemptFinancingsDetails" xlink:type="simple" xlink:href="cwst-20260331.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_019ce26e-a34f-72a7-8d84-0c0469ed5111" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019ce26e-a34f-72d7-8383-95c957a88bf1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ce26e-a34f-72a7-8d84-0c0469ed5111" xlink:to="loc_us-gaap_DebtInstrumentTable_019ce26e-a34f-72d7-8383-95c957a88bf1" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019ce26e-a34f-74b5-a15b-99cac03a1027" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019ce26e-a34f-72d7-8383-95c957a88bf1" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019ce26e-a34f-74b5-a15b-99cac03a1027" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019ce26e-a34f-74b5-a15b-99cac03a1027_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019ce26e-a34f-74b5-a15b-99cac03a1027" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019ce26e-a34f-74b5-a15b-99cac03a1027_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019ce26e-a34f-71a8-b713-98a1931f2abc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019ce26e-a34f-74b5-a15b-99cac03a1027" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019ce26e-a34f-71a8-b713-98a1931f2abc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnsecuredDebtMember_019ce26e-a34f-702a-9b26-310b6885034e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_UnsecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019ce26e-a34f-71a8-b713-98a1931f2abc" xlink:to="loc_us-gaap_UnsecuredDebtMember_019ce26e-a34f-702a-9b26-310b6885034e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019ce26e-a34f-72fc-a33a-33a1213ca3e0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019ce26e-a34f-72d7-8383-95c957a88bf1" xlink:to="loc_us-gaap_DebtInstrumentAxis_019ce26e-a34f-72fc-a33a-33a1213ca3e0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019ce26e-a34f-72fc-a33a-33a1213ca3e0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019ce26e-a34f-72fc-a33a-33a1213ca3e0" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019ce26e-a34f-72fc-a33a-33a1213ca3e0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019ce26e-a34f-7c99-b67e-7d637f21a2aa" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019ce26e-a34f-72fc-a33a-33a1213ca3e0" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019ce26e-a34f-7c99-b67e-7d637f21a2aa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_VermontBondsSeries2022A2Due2052Member_019ce26e-a34f-7565-a847-ad7a2488fbed" xlink:href="cwst-20260331.xsd#cwst_VermontBondsSeries2022A2Due2052Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019ce26e-a34f-7c99-b67e-7d637f21a2aa" xlink:to="loc_cwst_VermontBondsSeries2022A2Due2052Member_019ce26e-a34f-7565-a847-ad7a2488fbed" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentCarryingAmount_019ce26e-a34f-72d1-a3f7-5be3f27f1da2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ce26e-a34f-72a7-8d84-0c0469ed5111" xlink:to="loc_us-gaap_DebtInstrumentCarryingAmount_019ce26e-a34f-72d1-a3f7-5be3f27f1da2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_019ce26e-a34f-751e-8bcd-71db69c3c0f5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ce26e-a34f-72a7-8d84-0c0469ed5111" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_019ce26e-a34f-751e-8bcd-71db69c3c0f5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019ce26e-a34f-7d04-8730-b039a595b8e0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ce26e-a34f-72a7-8d84-0c0469ed5111" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019ce26e-a34f-7d04-8730-b039a595b8e0" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/DEBTNarrativeCashFlowHedgesDetails" xlink:type="simple" xlink:href="cwst-20260331.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_019ce26e-a34f-7128-9569-8f26b268d175" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeTable_019ce26e-a34f-775e-b1db-7bd594b85cd2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DerivativeLineItems_019ce26e-a34f-7128-9569-8f26b268d175" xlink:to="loc_us-gaap_DerivativeTable_019ce26e-a34f-775e-b1db-7bd594b85cd2" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_019ce26e-a34f-7a7f-b312-cd67d13c4470" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeInstrumentRiskAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeTable_019ce26e-a34f-775e-b1db-7bd594b85cd2" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_019ce26e-a34f-7a7f-b312-cd67d13c4470" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_019ce26e-a34f-7a7f-b312-cd67d13c4470_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_019ce26e-a34f-7a7f-b312-cd67d13c4470" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_019ce26e-a34f-7a7f-b312-cd67d13c4470_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_019ce26e-a34f-7cf9-aeaf-50dc71f7f6fd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_019ce26e-a34f-7a7f-b312-cd67d13c4470" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_019ce26e-a34f-7cf9-aeaf-50dc71f7f6fd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestRateContractMember_019ce26e-a34f-79c9-ba4e-7d8b8883a50e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestRateContractMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_019ce26e-a34f-7cf9-aeaf-50dc71f7f6fd" xlink:to="loc_us-gaap_InterestRateContractMember_019ce26e-a34f-79c9-ba4e-7d8b8883a50e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_019ce26e-a34f-77e1-8f9b-b746ee633de7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeTable_019ce26e-a34f-775e-b1db-7bd594b85cd2" xlink:to="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_019ce26e-a34f-77e1-8f9b-b746ee633de7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingRelationshipDomain_019ce26e-a34f-77e1-8f9b-b746ee633de7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HedgingRelationshipDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_019ce26e-a34f-77e1-8f9b-b746ee633de7" xlink:to="loc_us-gaap_HedgingRelationshipDomain_019ce26e-a34f-77e1-8f9b-b746ee633de7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingRelationshipDomain_019ce26e-a34f-703e-a98f-852873832b95" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HedgingRelationshipDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_019ce26e-a34f-77e1-8f9b-b746ee633de7" xlink:to="loc_us-gaap_HedgingRelationshipDomain_019ce26e-a34f-703e-a98f-852873832b95" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashFlowHedgingMember_019ce26e-a34f-71b1-8456-e3d8f789e81d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashFlowHedgingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingRelationshipDomain_019ce26e-a34f-703e-a98f-852873832b95" xlink:to="loc_us-gaap_CashFlowHedgingMember_019ce26e-a34f-71b1-8456-e3d8f789e81d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeNotionalAmount_019ce26e-a34f-7ecf-ac72-f5fc592d9b5e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeNotionalAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_019ce26e-a34f-7128-9569-8f26b268d175" xlink:to="loc_us-gaap_DerivativeNotionalAmount_019ce26e-a34f-7ecf-ac72-f5fc592d9b5e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeFloorInterestRate_019ce26e-a34f-7325-b543-bfc3731860d5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeFloorInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_019ce26e-a34f-7128-9569-8f26b268d175" xlink:to="loc_us-gaap_DerivativeFloorInterestRate_019ce26e-a34f-7325-b543-bfc3731860d5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeAverageFixedInterestRate_019ce26e-a34f-720d-b0ea-6549bbde4a0f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeAverageFixedInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_019ce26e-a34f-7128-9569-8f26b268d175" xlink:to="loc_us-gaap_DerivativeAverageFixedInterestRate_019ce26e-a34f-720d-b0ea-6549bbde4a0f" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/DEBTScheduleofCashFlowHedgesDetails" xlink:type="simple" xlink:href="cwst-20260331.xsd#DEBTScheduleofCashFlowHedgesDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/DEBTScheduleofCashFlowHedgesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativesFairValueLineItems_019ce26e-a34f-71f7-be16-89c373960808" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativesFairValueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_019ce26e-a34f-700b-80b7-3e0f38829b29" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_019ce26e-a34f-71f7-be16-89c373960808" xlink:to="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_019ce26e-a34f-700b-80b7-3e0f38829b29" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationAxis_019ce26e-a34f-7f72-9eb4-f331e113446c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BalanceSheetLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_019ce26e-a34f-700b-80b7-3e0f38829b29" xlink:to="loc_us-gaap_BalanceSheetLocationAxis_019ce26e-a34f-7f72-9eb4-f331e113446c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_019ce26e-a34f-7f72-9eb4-f331e113446c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BalanceSheetLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_019ce26e-a34f-7f72-9eb4-f331e113446c" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_019ce26e-a34f-7f72-9eb4-f331e113446c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_019ce26e-a34f-7368-bf7b-c64963c1de70" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BalanceSheetLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_019ce26e-a34f-7f72-9eb4-f331e113446c" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_019ce26e-a34f-7368-bf7b-c64963c1de70" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCurrentAssetsMember_019ce26e-a34f-7188-a0dc-6a3321e4c670" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherCurrentAssetsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_019ce26e-a34f-7368-bf7b-c64963c1de70" xlink:to="loc_us-gaap_OtherCurrentAssetsMember_019ce26e-a34f-7188-a0dc-6a3321e4c670" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNoncurrentAssetsMember_019ce26e-a34f-764a-a4bf-f463944459ea" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherNoncurrentAssetsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_019ce26e-a34f-7368-bf7b-c64963c1de70" xlink:to="loc_us-gaap_OtherNoncurrentAssetsMember_019ce26e-a34f-764a-a4bf-f463944459ea" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_OtherAccruedLiabilitiesMember_019ce26e-a34f-7243-8db7-a420e3de0c22" xlink:href="cwst-20260331.xsd#cwst_OtherAccruedLiabilitiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_019ce26e-a34f-7368-bf7b-c64963c1de70" xlink:to="loc_cwst_OtherAccruedLiabilitiesMember_019ce26e-a34f-7243-8db7-a420e3de0c22" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNoncurrentLiabilitiesMember_019ce26e-a34f-70dc-ace0-8f596280f4cf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherNoncurrentLiabilitiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_019ce26e-a34f-7368-bf7b-c64963c1de70" xlink:to="loc_us-gaap_OtherNoncurrentLiabilitiesMember_019ce26e-a34f-70dc-ace0-8f596280f4cf" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationAxis_019ce26e-a34f-7979-892c-70492cebbe43" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HedgingDesignationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_019ce26e-a34f-700b-80b7-3e0f38829b29" xlink:to="loc_us-gaap_HedgingDesignationAxis_019ce26e-a34f-7979-892c-70492cebbe43" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationDomain_019ce26e-a34f-7979-892c-70492cebbe43_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HedgingDesignationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_HedgingDesignationAxis_019ce26e-a34f-7979-892c-70492cebbe43" xlink:to="loc_us-gaap_HedgingDesignationDomain_019ce26e-a34f-7979-892c-70492cebbe43_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationDomain_019ce26e-a34f-71df-88e7-25ef28c1792f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HedgingDesignationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_HedgingDesignationAxis_019ce26e-a34f-7979-892c-70492cebbe43" xlink:to="loc_us-gaap_HedgingDesignationDomain_019ce26e-a34f-71df-88e7-25ef28c1792f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DesignatedAsHedgingInstrumentMember_019ce26e-a34f-7ffd-84b8-6d75f3ca366c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DesignatedAsHedgingInstrumentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingDesignationDomain_019ce26e-a34f-71df-88e7-25ef28c1792f" xlink:to="loc_us-gaap_DesignatedAsHedgingInstrumentMember_019ce26e-a34f-7ffd-84b8-6d75f3ca366c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_019ce26e-a34f-7bbd-b795-0b10a2b9aae7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeInstrumentRiskAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_019ce26e-a34f-700b-80b7-3e0f38829b29" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_019ce26e-a34f-7bbd-b795-0b10a2b9aae7" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_019ce26e-a34f-7bbd-b795-0b10a2b9aae7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_019ce26e-a34f-7bbd-b795-0b10a2b9aae7" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_019ce26e-a34f-7bbd-b795-0b10a2b9aae7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_019ce26e-a34f-7ede-b3b6-34574b836388" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_019ce26e-a34f-7bbd-b795-0b10a2b9aae7" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_019ce26e-a34f-7ede-b3b6-34574b836388" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestRateSwapMember_019ce26e-a34f-70d5-8dd2-0f361ded2471" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestRateSwapMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_019ce26e-a34f-7ede-b3b6-34574b836388" xlink:to="loc_us-gaap_InterestRateSwapMember_019ce26e-a34f-70d5-8dd2-0f361ded2471" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeFairValueOfDerivativeAsset_019ce26e-a34f-753a-9e2f-93b026213be9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeFairValueOfDerivativeAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_019ce26e-a34f-71f7-be16-89c373960808" xlink:to="loc_us-gaap_DerivativeFairValueOfDerivativeAsset_019ce26e-a34f-753a-9e2f-93b026213be9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeFairValueOfDerivativeLiability_019ce26e-a34f-7f35-a7a1-1bd08c4c5b85" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeFairValueOfDerivativeLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_019ce26e-a34f-71f7-be16-89c373960808" xlink:to="loc_us-gaap_DerivativeFairValueOfDerivativeLiability_019ce26e-a34f-7f35-a7a1-1bd08c4c5b85" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AociBeforeTaxAttributableToParent_019ce26e-a34f-755f-96ac-1333179747e9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AociBeforeTaxAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_019ce26e-a34f-71f7-be16-89c373960808" xlink:to="loc_us-gaap_AociBeforeTaxAttributableToParent_019ce26e-a34f-755f-96ac-1333179747e9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AociTaxAttributableToParent_019ce26e-a34f-7ff7-917f-24912e7207ec" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AociTaxAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_019ce26e-a34f-71f7-be16-89c373960808" xlink:to="loc_us-gaap_AociTaxAttributableToParent_019ce26e-a34f-7ff7-917f-24912e7207ec" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_019ce26e-a34f-7e20-b4aa-e7a519cd87af" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_019ce26e-a34f-71f7-be16-89c373960808" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_019ce26e-a34f-7e20-b4aa-e7a519cd87af" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/COMMITMENTSANDCONTINGENCIESNarrativeDetails" xlink:type="simple" xlink:href="cwst-20260331.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_019ce26e-a34f-7d51-b6fa-191daa110019" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LossContingenciesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesTable_019ce26e-a34f-7d9f-b483-f489a6a8aea3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LossContingenciesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LossContingenciesLineItems_019ce26e-a34f-7d51-b6fa-191daa110019" xlink:to="loc_us-gaap_LossContingenciesTable_019ce26e-a34f-7d9f-b483-f489a6a8aea3" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_LitigationCaseAxis_019ce26e-a34f-7a69-ac99-e52ded3d0d92" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_LitigationCaseAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_019ce26e-a34f-7d9f-b483-f489a6a8aea3" xlink:to="loc_srt_LitigationCaseAxis_019ce26e-a34f-7a69-ac99-e52ded3d0d92" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_LitigationCaseTypeDomain_019ce26e-a34f-7a69-ac99-e52ded3d0d92_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_LitigationCaseTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_LitigationCaseAxis_019ce26e-a34f-7a69-ac99-e52ded3d0d92" xlink:to="loc_srt_LitigationCaseTypeDomain_019ce26e-a34f-7a69-ac99-e52ded3d0d92_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_LitigationCaseTypeDomain_019ce26e-a34f-768a-a392-baf97e9983b0" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_LitigationCaseTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_LitigationCaseAxis_019ce26e-a34f-7a69-ac99-e52ded3d0d92" xlink:to="loc_srt_LitigationCaseTypeDomain_019ce26e-a34f-768a-a392-baf97e9983b0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_NewHampshireDepartmentOfEnvironmentalServicesLetterOfDeficiencyMember_019ce26e-a34f-7e85-8b7b-db1f7e5c6982" xlink:href="cwst-20260331.xsd#cwst_NewHampshireDepartmentOfEnvironmentalServicesLetterOfDeficiencyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_LitigationCaseTypeDomain_019ce26e-a34f-768a-a392-baf97e9983b0" xlink:to="loc_cwst_NewHampshireDepartmentOfEnvironmentalServicesLetterOfDeficiencyMember_019ce26e-a34f-7e85-8b7b-db1f7e5c6982" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingencyAccrualAtCarryingValue_019ce26e-a34f-7b98-a960-14e9cbb2cab7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LossContingencyAccrualAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019ce26e-a34f-7d51-b6fa-191daa110019" xlink:to="loc_us-gaap_LossContingencyAccrualAtCarryingValue_019ce26e-a34f-7b98-a960-14e9cbb2cab7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LitigationSettlementAmountAwardedToOtherParty_019d8b80-1f48-76a9-8150-ff9c79248781" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LitigationSettlementAmountAwardedToOtherParty"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019ce26e-a34f-7d51-b6fa-191daa110019" xlink:to="loc_us-gaap_LitigationSettlementAmountAwardedToOtherParty_019d8b80-1f48-76a9-8150-ff9c79248781" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredCosts_019ce26e-a34f-7477-ab62-f7e43810a79c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019ce26e-a34f-7d51-b6fa-191daa110019" xlink:to="loc_us-gaap_DeferredCosts_019ce26e-a34f-7477-ab62-f7e43810a79c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccrualForEnvironmentalLossContingenciesDiscountRate_019ce26e-a34f-7238-8d48-8e88e014354e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccrualForEnvironmentalLossContingenciesDiscountRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019ce26e-a34f-7d51-b6fa-191daa110019" xlink:to="loc_us-gaap_AccrualForEnvironmentalLossContingenciesDiscountRate_019ce26e-a34f-7238-8d48-8e88e014354e" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/STOCKHOLDERSEQUITYNarrativeDetails" xlink:type="simple" xlink:href="cwst-20260331.xsd#STOCKHOLDERSEQUITYNarrativeDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/STOCKHOLDERSEQUITYNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_019ce26e-a34f-74c5-a16d-34b5f223df30" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LimitedPartnersCapitalAccountLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfLimitedPartnersCapitalAccountByClassTable_019ce26e-a34f-79ce-b63b-a9e0c47c61b5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfLimitedPartnersCapitalAccountByClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_019ce26e-a34f-74c5-a16d-34b5f223df30" xlink:to="loc_us-gaap_ScheduleOfLimitedPartnersCapitalAccountByClassTable_019ce26e-a34f-79ce-b63b-a9e0c47c61b5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_019ce26e-a34f-7cc1-8e90-49bc23556618" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfLimitedPartnersCapitalAccountByClassTable_019ce26e-a34f-79ce-b63b-a9e0c47c61b5" xlink:to="loc_us-gaap_StatementClassOfStockAxis_019ce26e-a34f-7cc1-8e90-49bc23556618" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019ce26e-a34f-7cc1-8e90-49bc23556618_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019ce26e-a34f-7cc1-8e90-49bc23556618" xlink:to="loc_us-gaap_ClassOfStockDomain_019ce26e-a34f-7cc1-8e90-49bc23556618_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019ce26e-a34f-7d0b-9f26-a30e33f400d9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019ce26e-a34f-7cc1-8e90-49bc23556618" xlink:to="loc_us-gaap_ClassOfStockDomain_019ce26e-a34f-7d0b-9f26-a30e33f400d9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_019ce26e-a34f-760e-bf37-e64ae0b379b7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019ce26e-a34f-7d0b-9f26-a30e33f400d9" xlink:to="loc_us-gaap_CommonClassAMember_019ce26e-a34f-760e-bf37-e64ae0b379b7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_019ce26e-a34f-70c2-bb3c-fe0c9ec629f5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfLimitedPartnersCapitalAccountByClassTable_019ce26e-a34f-79ce-b63b-a9e0c47c61b5" xlink:to="loc_us-gaap_PlanNameAxis_019ce26e-a34f-70c2-bb3c-fe0c9ec629f5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_019ce26e-a34f-70c2-bb3c-fe0c9ec629f5_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_019ce26e-a34f-70c2-bb3c-fe0c9ec629f5" xlink:to="loc_us-gaap_PlanNameDomain_019ce26e-a34f-70c2-bb3c-fe0c9ec629f5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_019ce26e-a34f-7a7e-94f8-5a363e544015" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_019ce26e-a34f-70c2-bb3c-fe0c9ec629f5" xlink:to="loc_us-gaap_PlanNameDomain_019ce26e-a34f-7a7e-94f8-5a363e544015" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_Amended2016PlanMember_019ce26e-a34f-7295-ae7f-d138bc501304" xlink:href="cwst-20260331.xsd#cwst_Amended2016PlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_019ce26e-a34f-7a7e-94f8-5a363e544015" xlink:to="loc_cwst_Amended2016PlanMember_019ce26e-a34f-7295-ae7f-d138bc501304" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_A2016IncentivePlanMember_019ce26e-a34f-78e8-9d95-132afb2f6060" xlink:href="cwst-20260331.xsd#cwst_A2016IncentivePlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_019ce26e-a34f-7a7e-94f8-5a363e544015" xlink:to="loc_cwst_A2016IncentivePlanMember_019ce26e-a34f-78e8-9d95-132afb2f6060" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_A2006IncentivePlanMember_019ce26e-a34f-74e6-957f-32e50aaa8e5a" xlink:href="cwst-20260331.xsd#cwst_A2006IncentivePlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_019ce26e-a34f-7a7e-94f8-5a363e544015" xlink:to="loc_cwst_A2006IncentivePlanMember_019ce26e-a34f-74e6-957f-32e50aaa8e5a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_AmendedAndRestatedNineteenNinetySevenEmployeeStockPurchasePlanMember_019ce26e-a34f-7a29-b731-5f5efaa4a04b" xlink:href="cwst-20260331.xsd#cwst_AmendedAndRestatedNineteenNinetySevenEmployeeStockPurchasePlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_019ce26e-a34f-7a7e-94f8-5a363e544015" xlink:to="loc_cwst_AmendedAndRestatedNineteenNinetySevenEmployeeStockPurchasePlanMember_019ce26e-a34f-7a29-b731-5f5efaa4a04b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019ce26e-a34f-762d-94d4-e857f33b5504" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfLimitedPartnersCapitalAccountByClassTable_019ce26e-a34f-79ce-b63b-a9e0c47c61b5" xlink:to="loc_srt_RangeAxis_019ce26e-a34f-762d-94d4-e857f33b5504" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019ce26e-a34f-762d-94d4-e857f33b5504_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019ce26e-a34f-762d-94d4-e857f33b5504" xlink:to="loc_srt_RangeMember_019ce26e-a34f-762d-94d4-e857f33b5504_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019ce26e-a34f-79f5-bc22-6feed05c1385" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019ce26e-a34f-762d-94d4-e857f33b5504" xlink:to="loc_srt_RangeMember_019ce26e-a34f-79f5-bc22-6feed05c1385" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019ce26e-a34f-74af-af6d-af15b744e99a" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019ce26e-a34f-79f5-bc22-6feed05c1385" xlink:to="loc_srt_MinimumMember_019ce26e-a34f-74af-af6d-af15b744e99a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019ce26e-a34f-731e-ab91-c1ec78f8ed35" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019ce26e-a34f-79f5-bc22-6feed05c1385" xlink:to="loc_srt_MaximumMember_019ce26e-a34f-731e-ab91-c1ec78f8ed35" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_WeightedAverageMember_019ce26e-a34f-75d9-97fd-3ca7200d9c39" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_WeightedAverageMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019ce26e-a34f-79f5-bc22-6feed05c1385" xlink:to="loc_srt_WeightedAverageMember_019ce26e-a34f-75d9-97fd-3ca7200d9c39" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019ce26e-a34f-78d6-8488-1011d45cf747" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfLimitedPartnersCapitalAccountByClassTable_019ce26e-a34f-79ce-b63b-a9e0c47c61b5" xlink:to="loc_us-gaap_AwardTypeAxis_019ce26e-a34f-78d6-8488-1011d45cf747" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019ce26e-a34f-78d6-8488-1011d45cf747_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019ce26e-a34f-78d6-8488-1011d45cf747" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019ce26e-a34f-78d6-8488-1011d45cf747_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019ce26e-a34f-7763-b590-36da2dcdd3d0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019ce26e-a34f-78d6-8488-1011d45cf747" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019ce26e-a34f-7763-b590-36da2dcdd3d0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_019ce26e-a34f-7694-b9c1-d2fdd7b30733" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019ce26e-a34f-7763-b590-36da2dcdd3d0" xlink:to="loc_us-gaap_EmployeeStockOptionMember_019ce26e-a34f-7694-b9c1-d2fdd7b30733" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_RestrictedStockAwardsMember_019ce26e-a34f-7bb1-a51d-ae65cd328658" xlink:href="cwst-20260331.xsd#cwst_RestrictedStockAwardsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019ce26e-a34f-7763-b590-36da2dcdd3d0" xlink:to="loc_cwst_RestrictedStockAwardsMember_019ce26e-a34f-7bb1-a51d-ae65cd328658" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_019ce26e-a34f-73b8-86f5-a11eb241fa39" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019ce26e-a34f-7763-b590-36da2dcdd3d0" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_019ce26e-a34f-73b8-86f5-a11eb241fa39" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_RestrictedStockRestrictedStockUnitsAndPerformanceStockUnitsMember_019ce26e-a34f-79d8-a797-a4536cd5eb67" xlink:href="cwst-20260331.xsd#cwst_RestrictedStockRestrictedStockUnitsAndPerformanceStockUnitsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019ce26e-a34f-7763-b590-36da2dcdd3d0" xlink:to="loc_cwst_RestrictedStockRestrictedStockUnitsAndPerformanceStockUnitsMember_019ce26e-a34f-79d8-a797-a4536cd5eb67" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_PerformanceStockUnitMember_019ce26e-a34f-7540-802f-abb0e543889d" xlink:href="cwst-20260331.xsd#cwst_PerformanceStockUnitMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019ce26e-a34f-7763-b590-36da2dcdd3d0" xlink:to="loc_cwst_PerformanceStockUnitMember_019ce26e-a34f-7540-802f-abb0e543889d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_MarketbasedPerformanceStockUnitsMember_019ce26e-a34f-7e65-a91a-df9ecb7e8ed4" xlink:href="cwst-20260331.xsd#cwst_MarketbasedPerformanceStockUnitsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019ce26e-a34f-7763-b590-36da2dcdd3d0" xlink:to="loc_cwst_MarketbasedPerformanceStockUnitsMember_019ce26e-a34f-7e65-a91a-df9ecb7e8ed4" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualAxis_019ce26e-a34f-7990-855d-af3a384d04c4" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_TitleOfIndividualAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfLimitedPartnersCapitalAccountByClassTable_019ce26e-a34f-79ce-b63b-a9e0c47c61b5" xlink:to="loc_srt_TitleOfIndividualAxis_019ce26e-a34f-7990-855d-af3a384d04c4" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019ce26e-a34f-7990-855d-af3a384d04c4_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_TitleOfIndividualAxis_019ce26e-a34f-7990-855d-af3a384d04c4" xlink:to="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019ce26e-a34f-7990-855d-af3a384d04c4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019ce26e-a34f-7f45-bfbc-1a86922e06f5" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_TitleOfIndividualAxis_019ce26e-a34f-7990-855d-af3a384d04c4" xlink:to="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019ce26e-a34f-7f45-bfbc-1a86922e06f5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_NonEmployeeDirectorMember_019ce26e-a34f-71b3-8674-f2afd1eededa" xlink:href="cwst-20260331.xsd#cwst_NonEmployeeDirectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019ce26e-a34f-7f45-bfbc-1a86922e06f5" xlink:to="loc_cwst_NonEmployeeDirectorMember_019ce26e-a34f-71b3-8674-f2afd1eededa" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_019ce26e-a34f-7c23-a9a3-4bf1c87e3d3c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_019ce26e-a34f-74c5-a16d-34b5f223df30" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_019ce26e-a34f-7c23-a9a3-4bf1c87e3d3c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized_019ce26e-a34f-7b6d-82ce-cec90d690685" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_019ce26e-a34f-74c5-a16d-34b5f223df30" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized_019ce26e-a34f-7b6d-82ce-cec90d690685" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_019ce26e-a34f-766c-9b55-3e73263ad743" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_019ce26e-a34f-74c5-a16d-34b5f223df30" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_019ce26e-a34f-766c-9b55-3e73263ad743" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_019ce26e-a34f-72c8-b918-74c18f60900b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_019ce26e-a34f-74c5-a16d-34b5f223df30" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_019ce26e-a34f-72c8-b918-74c18f60900b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_019ce26e-a34f-7f62-b5ca-f14bb215af67" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_019ce26e-a34f-74c5-a16d-34b5f223df30" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_019ce26e-a34f-7f62-b5ca-f14bb215af67" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_019ce26e-a34f-7594-9fb8-4cf16a588a21" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_019ce26e-a34f-74c5-a16d-34b5f223df30" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_019ce26e-a34f-7594-9fb8-4cf16a588a21" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_019ce26e-a34f-7d2a-905d-6e77beb65a1b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_019ce26e-a34f-74c5-a16d-34b5f223df30" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_019ce26e-a34f-7d2a-905d-6e77beb65a1b" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_019ce26e-a34f-71a2-bbce-cfb41d8be488" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_019ce26e-a34f-74c5-a16d-34b5f223df30" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_019ce26e-a34f-71a2-bbce-cfb41d8be488" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue_019ce26e-a34f-7d4e-a471-0e8876a9d14d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_019ce26e-a34f-74c5-a16d-34b5f223df30" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue_019ce26e-a34f-7d4e-a471-0e8876a9d14d" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardRequisiteServicePeriod1_019ce26e-a34f-72ad-bee4-5f5f0ce51390" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardRequisiteServicePeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_019ce26e-a34f-74c5-a16d-34b5f223df30" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardRequisiteServicePeriod1_019ce26e-a34f-72ad-bee4-5f5f0ce51390" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue_019ce26e-a34f-7171-869d-5f9804e39c27" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_019ce26e-a34f-74c5-a16d-34b5f223df30" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue_019ce26e-a34f-7171-869d-5f9804e39c27" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_019ce26e-a34f-76c8-b2bf-5a5dc3af0b4e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_019ce26e-a34f-74c5-a16d-34b5f223df30" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_019ce26e-a34f-76c8-b2bf-5a5dc3af0b4e" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019ce26e-a34f-7203-aac7-4822e365cb7f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_019ce26e-a34f-74c5-a16d-34b5f223df30" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019ce26e-a34f-7203-aac7-4822e365cb7f" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_019ce26e-a34f-7895-b18d-6603c969dc2f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_019ce26e-a34f-74c5-a16d-34b5f223df30" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_019ce26e-a34f-7895-b18d-6603c969dc2f" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_019ce26e-a34f-7709-a276-6c1496947580" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_019ce26e-a34f-74c5-a16d-34b5f223df30" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_019ce26e-a34f-7709-a276-6c1496947580" xlink:type="arc" order="14"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/STOCKHOLDERSEQUITYScheduleofRestrictedStockAwardsRestrictedStockUnitandPerformancebasedStockUnitActivityDetails" xlink:type="simple" xlink:href="cwst-20260331.xsd#STOCKHOLDERSEQUITYScheduleofRestrictedStockAwardsRestrictedStockUnitandPerformancebasedStockUnitActivityDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/STOCKHOLDERSEQUITYScheduleofRestrictedStockAwardsRestrictedStockUnitandPerformancebasedStockUnitActivityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ce26e-a34f-7178-b461-cfc43fab288f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019ce26e-a34f-7cc6-ac03-30aab54a17ab" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ce26e-a34f-7178-b461-cfc43fab288f" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019ce26e-a34f-7cc6-ac03-30aab54a17ab" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingAxis_019ce26e-a34f-7013-8682-81f77b234e37" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VestingAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019ce26e-a34f-7cc6-ac03-30aab54a17ab" xlink:to="loc_us-gaap_VestingAxis_019ce26e-a34f-7013-8682-81f77b234e37" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingDomain_019ce26e-a34f-7013-8682-81f77b234e37_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VestingDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VestingAxis_019ce26e-a34f-7013-8682-81f77b234e37" xlink:to="loc_us-gaap_VestingDomain_019ce26e-a34f-7013-8682-81f77b234e37_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingDomain_019ce26e-a34f-7387-825d-ba61a3654c3d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VestingDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VestingAxis_019ce26e-a34f-7013-8682-81f77b234e37" xlink:to="loc_us-gaap_VestingDomain_019ce26e-a34f-7387-825d-ba61a3654c3d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationAwardTrancheOneMember_019ce26e-a34f-7e68-a96b-e66d271fdb6e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationAwardTrancheOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_019ce26e-a34f-7387-825d-ba61a3654c3d" xlink:to="loc_us-gaap_ShareBasedCompensationAwardTrancheOneMember_019ce26e-a34f-7e68-a96b-e66d271fdb6e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019ce26e-a34f-7dcf-b828-06cd88508fb1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019ce26e-a34f-7cc6-ac03-30aab54a17ab" xlink:to="loc_us-gaap_AwardTypeAxis_019ce26e-a34f-7dcf-b828-06cd88508fb1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019ce26e-a34f-7dcf-b828-06cd88508fb1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019ce26e-a34f-7dcf-b828-06cd88508fb1" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019ce26e-a34f-7dcf-b828-06cd88508fb1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019ce26e-a34f-7aec-bb88-29f1c8401bf8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019ce26e-a34f-7dcf-b828-06cd88508fb1" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019ce26e-a34f-7aec-bb88-29f1c8401bf8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_RestrictedStockRestrictedStockUnitsAndPerformanceStockUnitsMember_019ce26e-a34f-7776-bd2c-4aa8a9b7427b" xlink:href="cwst-20260331.xsd#cwst_RestrictedStockRestrictedStockUnitsAndPerformanceStockUnitsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019ce26e-a34f-7aec-bb88-29f1c8401bf8" xlink:to="loc_cwst_RestrictedStockRestrictedStockUnitsAndPerformanceStockUnitsMember_019ce26e-a34f-7776-bd2c-4aa8a9b7427b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_PerformanceStockUnitMember_019ce26e-a34f-7b54-b556-b45f56dd3d66" xlink:href="cwst-20260331.xsd#cwst_PerformanceStockUnitMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cwst_RestrictedStockRestrictedStockUnitsAndPerformanceStockUnitsMember_019ce26e-a34f-7776-bd2c-4aa8a9b7427b" xlink:to="loc_cwst_PerformanceStockUnitMember_019ce26e-a34f-7b54-b556-b45f56dd3d66" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_019ce26e-a34f-7c55-bb2d-8610e42033b9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019ce26e-a34f-7cc6-ac03-30aab54a17ab" xlink:to="loc_us-gaap_StatementClassOfStockAxis_019ce26e-a34f-7c55-bb2d-8610e42033b9" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019ce26e-a34f-7c55-bb2d-8610e42033b9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019ce26e-a34f-7c55-bb2d-8610e42033b9" xlink:to="loc_us-gaap_ClassOfStockDomain_019ce26e-a34f-7c55-bb2d-8610e42033b9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019ce26e-a34f-72b2-8028-9b281c976d79" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019ce26e-a34f-7c55-bb2d-8610e42033b9" xlink:to="loc_us-gaap_ClassOfStockDomain_019ce26e-a34f-72b2-8028-9b281c976d79" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_019ce26e-a34f-7a7a-b82c-6a90bd4f4d7f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019ce26e-a34f-72b2-8028-9b281c976d79" xlink:to="loc_us-gaap_CommonClassAMember_019ce26e-a34f-7a7a-b82c-6a90bd4f4d7f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019ce26e-a34f-7deb-a9db-6732828a94f2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ce26e-a34f-7178-b461-cfc43fab288f" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019ce26e-a34f-7deb-a9db-6732828a94f2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019ce26e-a34f-705a-828d-2cb58c3a144b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019ce26e-a34f-7deb-a9db-6732828a94f2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019ce26e-a34f-705a-828d-2cb58c3a144b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019ce26e-a34f-75b9-85d0-ee156144e73f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019ce26e-a34f-7deb-a9db-6732828a94f2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019ce26e-a34f-75b9-85d0-ee156144e73f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_019ce26e-a34f-72ca-ba75-2ed5b5b6797e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019ce26e-a34f-7deb-a9db-6732828a94f2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_019ce26e-a34f-72ca-ba75-2ed5b5b6797e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_019ce26e-a34f-7734-a2e6-b18c910fbf50" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019ce26e-a34f-7deb-a9db-6732828a94f2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_019ce26e-a34f-7734-a2e6-b18c910fbf50" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019ce26e-a34f-7262-8576-c14a30eff638" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019ce26e-a34f-7318-8192-5a5a06396091" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ce26e-a34f-7178-b461-cfc43fab288f" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019ce26e-a34f-7318-8192-5a5a06396091" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019ce26e-a34f-7209-b538-a674e7608e10" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019ce26e-a34f-7318-8192-5a5a06396091" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019ce26e-a34f-7209-b538-a674e7608e10" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019ce26e-a34f-711b-a5fc-eb32b5092daa" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019ce26e-a34f-7318-8192-5a5a06396091" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019ce26e-a34f-711b-a5fc-eb32b5092daa" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_019ce26e-a34f-78f2-b10c-6065cfe1ba19" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019ce26e-a34f-7318-8192-5a5a06396091" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_019ce26e-a34f-78f2-b10c-6065cfe1ba19" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_019ce26e-a34f-74dc-b83b-0a45cd83f96f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019ce26e-a34f-7318-8192-5a5a06396091" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_019ce26e-a34f-74dc-b83b-0a45cd83f96f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019ce26e-a34f-7b38-b368-1a53206e168c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalDisclosuresAbstract_019ce26e-a34f-7e23-8b39-7ddb9a06f49e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalDisclosuresAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ce26e-a34f-7178-b461-cfc43fab288f" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalDisclosuresAbstract_019ce26e-a34f-7e23-8b39-7ddb9a06f49e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingWeightedAverageRemainingContractualTerms_019ce26e-a34f-71f3-b4bd-e1e0a5aed448" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingWeightedAverageRemainingContractualTerms"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalDisclosuresAbstract_019ce26e-a34f-7e23-8b39-7ddb9a06f49e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingWeightedAverageRemainingContractualTerms_019ce26e-a34f-71f3-b4bd-e1e0a5aed448" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueOutstanding_019ce26e-a34f-70ef-a3fc-6e1462fc09c4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalDisclosuresAbstract_019ce26e-a34f-7e23-8b39-7ddb9a06f49e" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueOutstanding_019ce26e-a34f-70ef-a3fc-6e1462fc09c4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_019ce26e-a34f-70d7-babf-9dd795e5682a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ce26e-a34f-7178-b461-cfc43fab288f" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_019ce26e-a34f-70d7-babf-9dd795e5682a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_AdditionalSharesIssuedMaximumAttainmentLevel_019ce26e-a34f-73d5-b171-069863c0b23b" xlink:href="cwst-20260331.xsd#cwst_AdditionalSharesIssuedMaximumAttainmentLevel"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ce26e-a34f-7178-b461-cfc43fab288f" xlink:to="loc_cwst_AdditionalSharesIssuedMaximumAttainmentLevel_019ce26e-a34f-73d5-b171-069863c0b23b" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/STOCKHOLDERSEQUITYScheduleofChangesinBalancesofEachComponentofAccumulatedOtherComprehensiveIncomeLossDetails" xlink:type="simple" xlink:href="cwst-20260331.xsd#STOCKHOLDERSEQUITYScheduleofChangesinBalancesofEachComponentofAccumulatedOtherComprehensiveIncomeLossDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/STOCKHOLDERSEQUITYScheduleofChangesinBalancesofEachComponentofAccumulatedOtherComprehensiveIncomeLossDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_019ce26e-a34f-73c1-8071-0162e116b45d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_019ce26e-a34f-7d92-91f7-fd66724b16a5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_019ce26e-a34f-73c1-8071-0162e116b45d" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_019ce26e-a34f-7d92-91f7-fd66724b16a5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019ce26e-a34f-7d15-943a-8ee2b562b748" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_019ce26e-a34f-7d92-91f7-fd66724b16a5" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019ce26e-a34f-7d15-943a-8ee2b562b748" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019ce26e-a34f-7d15-943a-8ee2b562b748_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019ce26e-a34f-7d15-943a-8ee2b562b748" xlink:to="loc_us-gaap_EquityComponentDomain_019ce26e-a34f-7d15-943a-8ee2b562b748_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019ce26e-a34f-7441-ac9c-040b1618c0ca" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019ce26e-a34f-7d15-943a-8ee2b562b748" xlink:to="loc_us-gaap_EquityComponentDomain_019ce26e-a34f-7441-ac9c-040b1618c0ca" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedGainLossNetCashFlowHedgeParentMember_019ce26e-a34f-7764-90b0-bb50ffb95088" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccumulatedGainLossNetCashFlowHedgeParentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019ce26e-a34f-7441-ac9c-040b1618c0ca" xlink:to="loc_us-gaap_AccumulatedGainLossNetCashFlowHedgeParentMember_019ce26e-a34f-7764-90b0-bb50ffb95088" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTaxAbstract_019ce26e-a34f-7b75-a2fb-5cf48ab38912" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTaxAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_019ce26e-a34f-73c1-8071-0162e116b45d" xlink:to="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTaxAbstract_019ce26e-a34f-7b75-a2fb-5cf48ab38912" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019ce26e-a34f-7e0b-a090-fe4714f8acef" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTaxAbstract_019ce26e-a34f-7b75-a2fb-5cf48ab38912" xlink:to="loc_us-gaap_StockholdersEquity_019ce26e-a34f-7e0b-a090-fe4714f8acef" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OciBeforeReclassificationsBeforeTaxAttributableToParent_019ce26e-a34f-70f7-a8f4-530d95066031" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OciBeforeReclassificationsBeforeTaxAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTaxAbstract_019ce26e-a34f-7b75-a2fb-5cf48ab38912" xlink:to="loc_us-gaap_OciBeforeReclassificationsBeforeTaxAttributableToParent_019ce26e-a34f-70f7-a8f4-530d95066031" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationFromAociCurrentPeriodBeforeTaxAttributableToParent_019ce26e-a34f-7639-ae66-e3aed37fdd44" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ReclassificationFromAociCurrentPeriodBeforeTaxAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTaxAbstract_019ce26e-a34f-7b75-a2fb-5cf48ab38912" xlink:to="loc_us-gaap_ReclassificationFromAociCurrentPeriodBeforeTaxAttributableToParent_019ce26e-a34f-7639-ae66-e3aed37fdd44" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1_019ce26e-a34f-7ed0-97ee-a866a06ba2bd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTaxAbstract_019ce26e-a34f-7b75-a2fb-5cf48ab38912" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1_019ce26e-a34f-7ed0-97ee-a866a06ba2bd" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_019ce26e-a34f-79d2-815d-52d903a36557" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTaxAbstract_019ce26e-a34f-7b75-a2fb-5cf48ab38912" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_019ce26e-a34f-79d2-815d-52d903a36557" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019ce26e-a34f-7170-8342-263e8f325a51" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquity"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/STOCKHOLDERSEQUITYScheduleofReclassificationsOutofAccumulatedOtherComprehensiveLossDetails" xlink:type="simple" xlink:href="cwst-20260331.xsd#STOCKHOLDERSEQUITYScheduleofReclassificationsOutofAccumulatedOtherComprehensiveLossDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/STOCKHOLDERSEQUITYScheduleofReclassificationsOutofAccumulatedOtherComprehensiveLossDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_019ce26e-a34f-704c-9320-1b1d1be7c566" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeTable_019ce26e-a34f-7132-961a-407fbca6ec9b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_019ce26e-a34f-704c-9320-1b1d1be7c566" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeTable_019ce26e-a34f-7132-961a-407fbca6ec9b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019ce26e-a34f-73b2-88ee-efd1f8a10413" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeTable_019ce26e-a34f-7132-961a-407fbca6ec9b" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019ce26e-a34f-73b2-88ee-efd1f8a10413" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019ce26e-a34f-73b2-88ee-efd1f8a10413_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019ce26e-a34f-73b2-88ee-efd1f8a10413" xlink:to="loc_us-gaap_EquityComponentDomain_019ce26e-a34f-73b2-88ee-efd1f8a10413_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019ce26e-a34f-75b0-a86e-b72df4bc92a9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019ce26e-a34f-73b2-88ee-efd1f8a10413" xlink:to="loc_us-gaap_EquityComponentDomain_019ce26e-a34f-75b0-a86e-b72df4bc92a9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedGainLossNetCashFlowHedgeParentMember_019ce26e-a34f-7594-893f-70e0dbd11136" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccumulatedGainLossNetCashFlowHedgeParentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019ce26e-a34f-75b0-a86e-b72df4bc92a9" xlink:to="loc_us-gaap_AccumulatedGainLossNetCashFlowHedgeParentMember_019ce26e-a34f-7594-893f-70e0dbd11136" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis_019ce26e-a34f-7f25-af42-55574aa507f4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeTable_019ce26e-a34f-7132-961a-407fbca6ec9b" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis_019ce26e-a34f-7f25-af42-55574aa507f4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_019ce26e-a34f-7f25-af42-55574aa507f4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis_019ce26e-a34f-7f25-af42-55574aa507f4" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_019ce26e-a34f-7f25-af42-55574aa507f4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_019ce26e-a34f-7d56-8836-adc60e9ee027" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis_019ce26e-a34f-7f25-af42-55574aa507f4" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_019ce26e-a34f-7d56-8836-adc60e9ee027" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember_019ce26e-a34f-7acc-a933-797e42fc770d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_019ce26e-a34f-7d56-8836-adc60e9ee027" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember_019ce26e-a34f-7acc-a933-797e42fc770d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseNonoperating_019ce26e-a3e5-715b-a6ef-3b2265f39b24" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestExpenseNonoperating"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_019ce26e-a34f-704c-9320-1b1d1be7c566" xlink:to="loc_us-gaap_InterestExpenseNonoperating_019ce26e-a3e5-715b-a6ef-3b2265f39b24" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_019ce26e-a3e5-7aba-9b13-bfe0b34f5a66" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_019ce26e-a34f-704c-9320-1b1d1be7c566" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_019ce26e-a3e5-7aba-9b13-bfe0b34f5a66" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_019ce26e-a3e5-7f1c-a722-9efa401fcc12" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxExpenseBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_019ce26e-a34f-704c-9320-1b1d1be7c566" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_019ce26e-a3e5-7f1c-a722-9efa401fcc12" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019ce26e-a3e5-7ee7-9ef9-e83405d61d46" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_019ce26e-a34f-704c-9320-1b1d1be7c566" xlink:to="loc_us-gaap_NetIncomeLoss_019ce26e-a3e5-7ee7-9ef9-e83405d61d46" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/LOSSPERSHAREDetails" xlink:type="simple" xlink:href="cwst-20260331.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_019ce26e-a3e5-7f90-a5e5-23212d0166c0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_019ce26e-a3e5-77a0-b8e9-029c4bb3f359" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_019ce26e-a3e5-7f90-a5e5-23212d0166c0" xlink:to="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_019ce26e-a3e5-77a0-b8e9-029c4bb3f359" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_019ce26e-a3e5-7f75-9435-9a0240eb2f23" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_019ce26e-a3e5-77a0-b8e9-029c4bb3f359" xlink:to="loc_us-gaap_StatementClassOfStockAxis_019ce26e-a3e5-7f75-9435-9a0240eb2f23" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019ce26e-a3e5-7f75-9435-9a0240eb2f23_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019ce26e-a3e5-7f75-9435-9a0240eb2f23" xlink:to="loc_us-gaap_ClassOfStockDomain_019ce26e-a3e5-7f75-9435-9a0240eb2f23_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019ce26e-a3e5-7568-8e65-bfef54f8bec8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019ce26e-a3e5-7f75-9435-9a0240eb2f23" xlink:to="loc_us-gaap_ClassOfStockDomain_019ce26e-a3e5-7568-8e65-bfef54f8bec8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_019ce26e-a3e5-7904-8a55-6373bb8c8b3c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019ce26e-a3e5-7568-8e65-bfef54f8bec8" xlink:to="loc_us-gaap_CommonClassAMember_019ce26e-a3e5-7904-8a55-6373bb8c8b3c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_019ce26e-a3e5-74e2-b08c-86f5c58516e8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019ce26e-a3e5-7568-8e65-bfef54f8bec8" xlink:to="loc_us-gaap_CommonClassBMember_019ce26e-a3e5-74e2-b08c-86f5c58516e8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersDilutedAbstract_019ce26e-a3e5-7541-9540-c1a3cdbf9cbd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLossAvailableToCommonStockholdersDilutedAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_019ce26e-a3e5-7f90-a5e5-23212d0166c0" xlink:to="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersDilutedAbstract_019ce26e-a3e5-7541-9540-c1a3cdbf9cbd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperations_019ce26e-a3e5-7946-b195-b61b945933a5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeLossFromContinuingOperations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersDilutedAbstract_019ce26e-a3e5-7541-9540-c1a3cdbf9cbd" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperations_019ce26e-a3e5-7946-b195-b61b945933a5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract_019ce26e-a3e5-78ae-a605-c83945c2a9ff" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_019ce26e-a3e5-7f90-a5e5-23212d0166c0" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract_019ce26e-a3e5-78ae-a605-c83945c2a9ff" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019ce26e-a3e5-74e8-a10a-621343a505de" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract_019ce26e-a3e5-78ae-a605-c83945c2a9ff" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_019ce26e-a3e5-74e8-a10a-621343a505de" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesIssuedBasic_019ce26e-a3e5-71c4-8c8c-92e369624dbf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberOfSharesIssuedBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract_019ce26e-a3e5-78ae-a605-c83945c2a9ff" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesIssuedBasic_019ce26e-a3e5-71c4-8c8c-92e369624dbf" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_019ce26e-a3e5-7bbe-b28f-bff4985159d7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_019ce26e-a3e5-7f90-a5e5-23212d0166c0" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_019ce26e-a3e5-7bbe-b28f-bff4985159d7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustmentAbstract_019ce26e-a3e5-731c-9f8c-039b8c528b4d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustmentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_019ce26e-a3e5-7f90-a5e5-23212d0166c0" xlink:to="loc_us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustmentAbstract_019ce26e-a3e5-731c-9f8c-039b8c528b4d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements_019ce26e-a3e5-7dd6-9064-478f97e830cc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustmentAbstract_019ce26e-a3e5-731c-9f8c-039b8c528b4d" xlink:to="loc_us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements_019ce26e-a3e5-7dd6-9064-478f97e830cc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_019ce26e-a3e5-7212-be17-d284a9767668" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_019ce26e-a3e5-7f90-a5e5-23212d0166c0" xlink:to="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_019ce26e-a3e5-7212-be17-d284a9767668" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_019ce26e-a3e5-7fc7-9426-5a2059cf0e75" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_019ce26e-a3e5-7f90-a5e5-23212d0166c0" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_019ce26e-a3e5-7fc7-9426-5a2059cf0e75" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/FAIRVALUEOFFINANCIALINSTRUMENTSScheduleofRecurringFairValueMeasurementsDetails" xlink:type="simple" xlink:href="cwst-20260331.xsd#FAIRVALUEOFFINANCIALINSTRUMENTSScheduleofRecurringFairValueMeasurementsDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/FAIRVALUEOFFINANCIALINSTRUMENTSScheduleofRecurringFairValueMeasurementsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019ce26e-a3e5-7d26-bcd2-24830cb8038c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019ce26e-a3e5-7184-bece-c4feb81fb74c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019ce26e-a3e5-7d26-bcd2-24830cb8038c" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019ce26e-a3e5-7184-bece-c4feb81fb74c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019ce26e-a3e5-74ca-b3dc-0caa4f81eb41" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019ce26e-a3e5-7184-bece-c4feb81fb74c" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019ce26e-a3e5-74ca-b3dc-0caa4f81eb41" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019ce26e-a3e5-74ca-b3dc-0caa4f81eb41_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019ce26e-a3e5-74ca-b3dc-0caa4f81eb41" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019ce26e-a3e5-74ca-b3dc-0caa4f81eb41_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019ce26e-a3e5-7783-8307-b22e5b4c772f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019ce26e-a3e5-74ca-b3dc-0caa4f81eb41" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019ce26e-a3e5-7783-8307-b22e5b4c772f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_019ce26e-a3e5-7e71-ab42-b71d1f101a3c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019ce26e-a3e5-7783-8307-b22e5b4c772f" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_019ce26e-a3e5-7e71-ab42-b71d1f101a3c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_019ce26e-a3e5-7807-bfed-5e76f2c54eb1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019ce26e-a3e5-7783-8307-b22e5b4c772f" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_019ce26e-a3e5-7807-bfed-5e76f2c54eb1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_019ce26e-a3e5-7169-950f-8bee043b2692" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019ce26e-a3e5-7783-8307-b22e5b4c772f" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_019ce26e-a3e5-7169-950f-8bee043b2692" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019ce26e-a3e5-786b-a0b1-483c35ef0faa" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019ce26e-a3e5-7184-bece-c4feb81fb74c" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019ce26e-a3e5-786b-a0b1-483c35ef0faa" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019ce26e-a3e5-786b-a0b1-483c35ef0faa_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019ce26e-a3e5-786b-a0b1-483c35ef0faa" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019ce26e-a3e5-786b-a0b1-483c35ef0faa_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019ce26e-a3e5-7c35-a21c-3d3b6b82ad77" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019ce26e-a3e5-786b-a0b1-483c35ef0faa" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019ce26e-a3e5-7c35-a21c-3d3b6b82ad77" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LandfillMember_019ce26e-a3e5-798a-9a80-525270d54d0e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LandfillMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019ce26e-a3e5-7c35-a21c-3d3b6b82ad77" xlink:to="loc_us-gaap_LandfillMember_019ce26e-a3e5-798a-9a80-525270d54d0e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_019de422-2490-77e5-9fad-5a168872ca94" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeInstrumentRiskAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019ce26e-a3e5-7184-bece-c4feb81fb74c" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_019de422-2490-77e5-9fad-5a168872ca94" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_019de422-2490-77e5-9fad-5a168872ca94_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_019de422-2490-77e5-9fad-5a168872ca94" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_019de422-2490-77e5-9fad-5a168872ca94_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_019de422-2490-77c4-ad47-58eece69f288" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_019de422-2490-77e5-9fad-5a168872ca94" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_019de422-2490-77c4-ad47-58eece69f288" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestRateSwapMember_019de422-551a-7c16-9a51-26c34ff24e4f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestRateSwapMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_019de422-2490-77c4-ad47-58eece69f288" xlink:to="loc_us-gaap_InterestRateSwapMember_019de422-551a-7c16-9a51-26c34ff24e4f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsAbstract_019ce26e-a3e5-735b-81cc-f959c5afe8d2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019ce26e-a3e5-7d26-bcd2-24830cb8038c" xlink:to="loc_us-gaap_AssetsAbstract_019ce26e-a3e5-735b-81cc-f959c5afe8d2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndInvestmentsNoncurrent_019ce26e-a3e5-71ab-8a58-707c1bb3bbaf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedCashAndInvestmentsNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019ce26e-a3e5-735b-81cc-f959c5afe8d2" xlink:to="loc_us-gaap_RestrictedCashAndInvestmentsNoncurrent_019ce26e-a3e5-71ab-8a58-707c1bb3bbaf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeAssets_019ce26e-a3e5-7c04-b5d0-781b7c84f5c3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019ce26e-a3e5-735b-81cc-f959c5afe8d2" xlink:to="loc_us-gaap_DerivativeAssets_019ce26e-a3e5-7c04-b5d0-781b7c84f5c3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_DerivativeAssetStatementOfFinancialPositionExtensibleEnumerationNotDisclosedFlag_019ce26e-a3e5-77df-8aa8-2fc89ad72f18" xlink:href="cwst-20260331.xsd#cwst_DerivativeAssetStatementOfFinancialPositionExtensibleEnumerationNotDisclosedFlag"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeAssets_019ce26e-a3e5-7c04-b5d0-781b7c84f5c3" xlink:to="loc_cwst_DerivativeAssetStatementOfFinancialPositionExtensibleEnumerationNotDisclosedFlag_019ce26e-a3e5-77df-8aa8-2fc89ad72f18" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsFairValueDisclosure_019ce26e-a3e5-7d7a-85c0-5ffa76f5d33e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019ce26e-a3e5-735b-81cc-f959c5afe8d2" xlink:to="loc_us-gaap_AssetsFairValueDisclosure_019ce26e-a3e5-7d7a-85c0-5ffa76f5d33e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAbstract_019ce26e-a3e5-70bf-ba07-a775dfd675ba" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019ce26e-a3e5-7d26-bcd2-24830cb8038c" xlink:to="loc_us-gaap_LiabilitiesAbstract_019ce26e-a3e5-70bf-ba07-a775dfd675ba" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestRateDerivativeLiabilitiesAtFairValue_019ce26e-a3e5-7c88-8805-014b68be51ee" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestRateDerivativeLiabilitiesAtFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAbstract_019ce26e-a3e5-70bf-ba07-a775dfd675ba" xlink:to="loc_us-gaap_InterestRateDerivativeLiabilitiesAtFairValue_019ce26e-a3e5-7c88-8805-014b68be51ee" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/FAIRVALUEOFFINANCIALINSTRUMENTSNarrativeDetails" xlink:type="simple" xlink:href="cwst-20260331.xsd#FAIRVALUEOFFINANCIALINSTRUMENTSNarrativeDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/FAIRVALUEOFFINANCIALINSTRUMENTSNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019ce26e-a3e5-7bc8-aa85-494b50afc3af" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019ce26e-a3e5-7c42-9b13-63f4cb300381" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ce26e-a3e5-7bc8-aa85-494b50afc3af" xlink:to="loc_us-gaap_DebtInstrumentTable_019ce26e-a3e5-7c42-9b13-63f4cb300381" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementBasisAxis_019ce26e-a3e5-794f-b362-ff444e08bbd5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByMeasurementBasisAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019ce26e-a3e5-7c42-9b13-63f4cb300381" xlink:to="loc_us-gaap_FairValueByMeasurementBasisAxis_019ce26e-a3e5-794f-b362-ff444e08bbd5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019ce26e-a3e5-794f-b362-ff444e08bbd5_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementBasisAxis_019ce26e-a3e5-794f-b362-ff444e08bbd5" xlink:to="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019ce26e-a3e5-794f-b362-ff444e08bbd5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueDisclosureItemAmountsDomain_019ce26e-a3e5-7044-866b-0d4ab877d50a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueDisclosureItemAmountsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementBasisAxis_019ce26e-a3e5-794f-b362-ff444e08bbd5" xlink:to="loc_us-gaap_FairValueDisclosureItemAmountsDomain_019ce26e-a3e5-7044-866b-0d4ab877d50a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019ce26e-a3e5-7921-b2c8-8b8748d30bcd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureItemAmountsDomain_019ce26e-a3e5-7044-866b-0d4ab877d50a" xlink:to="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019ce26e-a3e5-7921-b2c8-8b8748d30bcd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CarryingReportedAmountFairValueDisclosureMember_019ce26e-a3e5-734f-94b8-71634e862fc8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CarryingReportedAmountFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019ce26e-a3e5-7921-b2c8-8b8748d30bcd" xlink:to="loc_us-gaap_CarryingReportedAmountFairValueDisclosureMember_019ce26e-a3e5-734f-94b8-71634e862fc8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EstimateOfFairValueFairValueDisclosureMember_019ce26e-a3e5-7d9f-b0c9-340534746513" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EstimateOfFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019ce26e-a3e5-7921-b2c8-8b8748d30bcd" xlink:to="loc_us-gaap_EstimateOfFairValueFairValueDisclosureMember_019ce26e-a3e5-7d9f-b0c9-340534746513" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019ce26e-a3e5-7477-a258-54f718ccd867" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019ce26e-a3e5-7c42-9b13-63f4cb300381" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019ce26e-a3e5-7477-a258-54f718ccd867" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019ce26e-a3e5-7477-a258-54f718ccd867_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019ce26e-a3e5-7477-a258-54f718ccd867" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019ce26e-a3e5-7477-a258-54f718ccd867_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019ce26e-a3e5-7d1c-b164-c8c7998d2bec" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019ce26e-a3e5-7477-a258-54f718ccd867" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019ce26e-a3e5-7d1c-b164-c8c7998d2bec" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_019ce26e-a3e5-7187-8e07-3e1ed974675f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019ce26e-a3e5-7d1c-b164-c8c7998d2bec" xlink:to="loc_us-gaap_SecuredDebtMember_019ce26e-a3e5-7187-8e07-3e1ed974675f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_019ce26e-a3e5-76dc-9337-0d8429eb4a42" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019ce26e-a3e5-7d1c-b164-c8c7998d2bec" xlink:to="loc_us-gaap_LineOfCreditMember_019ce26e-a3e5-76dc-9337-0d8429eb4a42" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnsecuredDebtMember_019ce26e-a3e5-72d8-a022-563e208fce1e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_UnsecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019ce26e-a3e5-7d1c-b164-c8c7998d2bec" xlink:to="loc_us-gaap_UnsecuredDebtMember_019ce26e-a3e5-72d8-a022-563e208fce1e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFairValue_019ce26e-a3e5-7012-8a44-b2347e62d509" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ce26e-a3e5-7bc8-aa85-494b50afc3af" xlink:to="loc_us-gaap_DebtInstrumentFairValue_019ce26e-a3e5-7012-8a44-b2347e62d509" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/SEGMENTREPORTINGNarrativeDetails" xlink:type="simple" xlink:href="cwst-20260331.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_019ce26e-a3e5-731a-a24e-9d67f5a1410c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019ce26e-a3e5-7dff-8ebe-e68ae0db52bf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ce26e-a3e5-731a-a24e-9d67f5a1410c" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019ce26e-a3e5-7dff-8ebe-e68ae0db52bf" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_019ce26e-a3e5-77c6-ad3d-178f0fb2a9f3" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019ce26e-a3e5-7dff-8ebe-e68ae0db52bf" xlink:to="loc_srt_ProductOrServiceAxis_019ce26e-a3e5-77c6-ad3d-178f0fb2a9f3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019ce26e-a3e5-77c6-ad3d-178f0fb2a9f3_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_019ce26e-a3e5-77c6-ad3d-178f0fb2a9f3" xlink:to="loc_srt_ProductsAndServicesDomain_019ce26e-a3e5-77c6-ad3d-178f0fb2a9f3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019ce26e-a3e5-7d75-a57f-9d815e213c80" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_019ce26e-a3e5-77c6-ad3d-178f0fb2a9f3" xlink:to="loc_srt_ProductsAndServicesDomain_019ce26e-a3e5-7d75-a57f-9d815e213c80" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_SolidWasteOperationsMember_019ce26e-a3e5-77a1-bbf5-1ce156b60b2e" xlink:href="cwst-20260331.xsd#cwst_SolidWasteOperationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019ce26e-a3e5-7d75-a57f-9d815e213c80" xlink:to="loc_cwst_SolidWasteOperationsMember_019ce26e-a3e5-77a1-bbf5-1ce156b60b2e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfOperatingSegments_019ce26e-a3e5-723b-9b23-f8e443d697a4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NumberOfOperatingSegments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ce26e-a3e5-731a-a24e-9d67f5a1410c" xlink:to="loc_us-gaap_NumberOfOperatingSegments_019ce26e-a3e5-723b-9b23-f8e443d697a4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfReportableSegments_019d8d86-1072-76f8-9e1e-602a1c43819a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NumberOfReportableSegments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ce26e-a3e5-731a-a24e-9d67f5a1410c" xlink:to="loc_us-gaap_NumberOfReportableSegments_019d8d86-1072-76f8-9e1e-602a1c43819a" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/SEGMENTREPORTINGScheduleofFinancialInformationbyReportableSegmentDetails" xlink:type="simple" xlink:href="cwst-20260331.xsd#SEGMENTREPORTINGScheduleofFinancialInformationbyReportableSegmentDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/SEGMENTREPORTINGScheduleofFinancialInformationbyReportableSegmentDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_019ce26e-a3e5-7eb2-882e-42254da7b4b5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019ce26e-a3e5-7a8b-8cd5-cfdd194b00cb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ce26e-a3e5-7eb2-882e-42254da7b4b5" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019ce26e-a3e5-7a8b-8cd5-cfdd194b00cb" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsAxis_019ce26e-a3e5-7553-8f64-a7b335bdc426" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ConsolidationItemsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019ce26e-a3e5-7a8b-8cd5-cfdd194b00cb" xlink:to="loc_srt_ConsolidationItemsAxis_019ce26e-a3e5-7553-8f64-a7b335bdc426" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_019ce26e-a3e5-7553-8f64-a7b335bdc426_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidationItemsAxis_019ce26e-a3e5-7553-8f64-a7b335bdc426" xlink:to="loc_srt_ConsolidationItemsDomain_019ce26e-a3e5-7553-8f64-a7b335bdc426_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_019ce26e-a3e5-72fc-8fde-1c41b07acd7c" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidationItemsAxis_019ce26e-a3e5-7553-8f64-a7b335bdc426" xlink:to="loc_srt_ConsolidationItemsDomain_019ce26e-a3e5-72fc-8fde-1c41b07acd7c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IntersegmentEliminationMember_019ce26e-a3e5-720d-a923-90f3fcb4c161" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IntersegmentEliminationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_019ce26e-a3e5-72fc-8fde-1c41b07acd7c" xlink:to="loc_us-gaap_IntersegmentEliminationMember_019ce26e-a3e5-720d-a923-90f3fcb4c161" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingSegmentsMember_019ce26e-a3e5-726c-8ca0-e883ec6396ce" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingSegmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_019ce26e-a3e5-72fc-8fde-1c41b07acd7c" xlink:to="loc_us-gaap_OperatingSegmentsMember_019ce26e-a3e5-726c-8ca0-e883ec6396ce" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateNonSegmentMember_019ce26e-a3e5-786b-ba18-fa76aef0c5d2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CorporateNonSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_019ce26e-a3e5-72fc-8fde-1c41b07acd7c" xlink:to="loc_us-gaap_CorporateNonSegmentMember_019ce26e-a3e5-786b-ba18-fa76aef0c5d2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019ce26e-a3e5-7838-a948-add7b59fca2a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019ce26e-a3e5-7a8b-8cd5-cfdd194b00cb" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019ce26e-a3e5-7838-a948-add7b59fca2a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019ce26e-a3e5-7838-a948-add7b59fca2a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019ce26e-a3e5-7838-a948-add7b59fca2a" xlink:to="loc_us-gaap_SegmentDomain_019ce26e-a3e5-7838-a948-add7b59fca2a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019ce26e-a3e5-7465-a83b-06690c1ad948" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019ce26e-a3e5-7838-a948-add7b59fca2a" xlink:to="loc_us-gaap_SegmentDomain_019ce26e-a3e5-7465-a83b-06690c1ad948" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_SolidWasteOperationsMember_019ce26e-a3e5-71ee-bdaa-206a5376f5a8" xlink:href="cwst-20260331.xsd#cwst_SolidWasteOperationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019ce26e-a3e5-7465-a83b-06690c1ad948" xlink:to="loc_cwst_SolidWasteOperationsMember_019ce26e-a3e5-71ee-bdaa-206a5376f5a8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_ResourceSolutionsRegionMember_019ce26e-a3e5-75ac-8c93-749acfe7d430" xlink:href="cwst-20260331.xsd#cwst_ResourceSolutionsRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019ce26e-a3e5-7465-a83b-06690c1ad948" xlink:to="loc_cwst_ResourceSolutionsRegionMember_019ce26e-a3e5-75ac-8c93-749acfe7d430" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsegmentsAxis_019ce26e-a3e5-70db-a89c-94f00079ab4e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019ce26e-a3e5-7a8b-8cd5-cfdd194b00cb" xlink:to="loc_us-gaap_SubsegmentsAxis_019ce26e-a3e5-70db-a89c-94f00079ab4e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsegmentsDomain_019ce26e-a3e5-70db-a89c-94f00079ab4e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsegmentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsegmentsAxis_019ce26e-a3e5-70db-a89c-94f00079ab4e" xlink:to="loc_us-gaap_SubsegmentsDomain_019ce26e-a3e5-70db-a89c-94f00079ab4e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsegmentsDomain_019ce26e-a3e5-7bcd-aafb-108d4884620a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsegmentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsegmentsAxis_019ce26e-a3e5-70db-a89c-94f00079ab4e" xlink:to="loc_us-gaap_SubsegmentsDomain_019ce26e-a3e5-7bcd-aafb-108d4884620a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_EasternRegionMember_019ce26e-a3e5-7c13-aa44-504fd1d7d1f5" xlink:href="cwst-20260331.xsd#cwst_EasternRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsegmentsDomain_019ce26e-a3e5-7bcd-aafb-108d4884620a" xlink:to="loc_cwst_EasternRegionMember_019ce26e-a3e5-7c13-aa44-504fd1d7d1f5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_WesternRegionMember_019ce26e-a3e5-7354-831a-d499e47e5c4c" xlink:href="cwst-20260331.xsd#cwst_WesternRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsegmentsDomain_019ce26e-a3e5-7bcd-aafb-108d4884620a" xlink:to="loc_cwst_WesternRegionMember_019ce26e-a3e5-7354-831a-d499e47e5c4c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_MidAtlanticRegionMember_019ce26e-a3e5-791d-bf31-3eb68acfe29a" xlink:href="cwst-20260331.xsd#cwst_MidAtlanticRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsegmentsDomain_019ce26e-a3e5-7bcd-aafb-108d4884620a" xlink:to="loc_cwst_MidAtlanticRegionMember_019ce26e-a3e5-791d-bf31-3eb68acfe29a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Revenues_019ce26e-a3e5-7d6e-8c0d-7aa1bda33a7c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Revenues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ce26e-a3e5-7eb2-882e-42254da7b4b5" xlink:to="loc_us-gaap_Revenues_019ce26e-a3e5-7d6e-8c0d-7aa1bda33a7c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfGoodsAndServicesSold_019ce26e-a3e5-78b9-800f-c897d094babb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CostOfGoodsAndServicesSold"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ce26e-a3e5-7eb2-882e-42254da7b4b5" xlink:to="loc_us-gaap_CostOfGoodsAndServicesSold_019ce26e-a3e5-78b9-800f-c897d094babb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GeneralAndAdministrativeExpense_019ce26e-a3e5-72ac-af1e-e043d94dd042" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GeneralAndAdministrativeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ce26e-a3e5-7eb2-882e-42254da7b4b5" xlink:to="loc_us-gaap_GeneralAndAdministrativeExpense_019ce26e-a3e5-72ac-af1e-e043d94dd042" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DepreciationDepletionAndAmortization_019ce26e-a3e5-7101-99b8-a495d667916e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DepreciationDepletionAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ce26e-a3e5-7eb2-882e-42254da7b4b5" xlink:to="loc_us-gaap_DepreciationDepletionAndAmortization_019ce26e-a3e5-7101-99b8-a495d667916e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationAcquisitionRelatedCosts_019de419-23bd-79e7-8638-bf3eb74f23ee" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationAcquisitionRelatedCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ce26e-a3e5-7eb2-882e-42254da7b4b5" xlink:to="loc_us-gaap_BusinessCombinationAcquisitionRelatedCosts_019de419-23bd-79e7-8638-bf3eb74f23ee" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_OrganicsFacilityClosureCharge_019d8b90-746f-7415-954d-4e4facc90dee" xlink:href="cwst-20260331.xsd#cwst_OrganicsFacilityClosureCharge"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ce26e-a3e5-7eb2-882e-42254da7b4b5" xlink:to="loc_cwst_OrganicsFacilityClosureCharge_019d8b90-746f-7415-954d-4e4facc90dee" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingIncomeLoss_019ce26e-a3e5-7b6e-b5bb-20ad1d78209f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ce26e-a3e5-7eb2-882e-42254da7b4b5" xlink:to="loc_us-gaap_OperatingIncomeLoss_019ce26e-a3e5-7b6e-b5bb-20ad1d78209f" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestIncomeExpenseNonoperatingNet_019ce26e-a3e5-7db9-8eab-45efb070a395" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestIncomeExpenseNonoperatingNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ce26e-a3e5-7eb2-882e-42254da7b4b5" xlink:to="loc_us-gaap_InterestIncomeExpenseNonoperatingNet_019ce26e-a3e5-7db9-8eab-45efb070a395" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNonoperatingIncomeExpense_019ce26e-a3e5-75c9-ad95-8a157c46b59f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherNonoperatingIncomeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ce26e-a3e5-7eb2-882e-42254da7b4b5" xlink:to="loc_us-gaap_OtherNonoperatingIncomeExpense_019ce26e-a3e5-75c9-ad95-8a157c46b59f" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_019ce26e-a3e5-7878-9312-e67808ba2c5b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ce26e-a3e5-7eb2-882e-42254da7b4b5" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_019ce26e-a3e5-7878-9312-e67808ba2c5b" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentExpenditureAdditionToLongLivedAssets_019ce26e-a3e5-7395-9845-dee3dce213f3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentExpenditureAdditionToLongLivedAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ce26e-a3e5-7eb2-882e-42254da7b4b5" xlink:to="loc_us-gaap_SegmentExpenditureAdditionToLongLivedAssets_019ce26e-a3e5-7395-9845-dee3dce213f3" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Assets_019ce26e-a3e5-76d9-bb15-e48018d8de39" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Assets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019ce26e-a3e5-7eb2-882e-42254da7b4b5" xlink:to="loc_us-gaap_Assets_019ce26e-a3e5-76d9-bb15-e48018d8de39" xlink:type="arc" order="11"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/SEGMENTREPORTINGScheduleofRevenuesAttributabletoServicesProvidedDetails" xlink:type="simple" xlink:href="cwst-20260331.xsd#SEGMENTREPORTINGScheduleofRevenuesAttributabletoServicesProvidedDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/SEGMENTREPORTINGScheduleofRevenuesAttributabletoServicesProvidedDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_019ce26e-a3e5-7ef7-95ce-d1b02b62289b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_019ce26e-a3e5-77c8-9c6e-1e5869c3645c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_019ce26e-a3e5-7ef7-95ce-d1b02b62289b" xlink:to="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_019ce26e-a3e5-77c8-9c6e-1e5869c3645c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_019ce26e-a3e5-7e6f-9092-86663be312d8" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_019ce26e-a3e5-77c8-9c6e-1e5869c3645c" xlink:to="loc_srt_ProductOrServiceAxis_019ce26e-a3e5-7e6f-9092-86663be312d8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019ce26e-a3e5-7e6f-9092-86663be312d8_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_019ce26e-a3e5-7e6f-9092-86663be312d8" xlink:to="loc_srt_ProductsAndServicesDomain_019ce26e-a3e5-7e6f-9092-86663be312d8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019ce26e-a3e5-74fe-b8fe-de3a817356b6" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_019ce26e-a3e5-7e6f-9092-86663be312d8" xlink:to="loc_srt_ProductsAndServicesDomain_019ce26e-a3e5-74fe-b8fe-de3a817356b6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_SolidWasteOperationsMember_019ce26e-a3e5-72e7-9b09-cef20de68e57" xlink:href="cwst-20260331.xsd#cwst_SolidWasteOperationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019ce26e-a3e5-74fe-b8fe-de3a817356b6" xlink:to="loc_cwst_SolidWasteOperationsMember_019ce26e-a3e5-72e7-9b09-cef20de68e57" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_SolidWasteOperationsCollectionMember_019ce26e-a3e5-7dd2-a902-ac8a1863f86a" xlink:href="cwst-20260331.xsd#cwst_SolidWasteOperationsCollectionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cwst_SolidWasteOperationsMember_019ce26e-a3e5-72e7-9b09-cef20de68e57" xlink:to="loc_cwst_SolidWasteOperationsCollectionMember_019ce26e-a3e5-7dd2-a902-ac8a1863f86a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_SolidWasteOperationsDisposalMember_019ce26e-a3e5-7160-a646-0d227ed68078" xlink:href="cwst-20260331.xsd#cwst_SolidWasteOperationsDisposalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cwst_SolidWasteOperationsMember_019ce26e-a3e5-72e7-9b09-cef20de68e57" xlink:to="loc_cwst_SolidWasteOperationsDisposalMember_019ce26e-a3e5-7160-a646-0d227ed68078" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_SolidWasteOperationsTransportationMember_019d9e70-b0b8-76a5-a589-620f097e4441" xlink:href="cwst-20260331.xsd#cwst_SolidWasteOperationsTransportationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cwst_SolidWasteOperationsMember_019ce26e-a3e5-72e7-9b09-cef20de68e57" xlink:to="loc_cwst_SolidWasteOperationsTransportationMember_019d9e70-b0b8-76a5-a589-620f097e4441" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_SolidWasteOperationsLandfillGasToEnergyMember_019ce26e-a3e5-76b0-97fa-e54fff92ab9a" xlink:href="cwst-20260331.xsd#cwst_SolidWasteOperationsLandfillGasToEnergyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cwst_SolidWasteOperationsMember_019ce26e-a3e5-72e7-9b09-cef20de68e57" xlink:to="loc_cwst_SolidWasteOperationsLandfillGasToEnergyMember_019ce26e-a3e5-76b0-97fa-e54fff92ab9a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_SolidWasteOperationsProcessingMember_019ce26e-a3e5-7543-b0bf-3c9c6b6b219c" xlink:href="cwst-20260331.xsd#cwst_SolidWasteOperationsProcessingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cwst_SolidWasteOperationsMember_019ce26e-a3e5-72e7-9b09-cef20de68e57" xlink:to="loc_cwst_SolidWasteOperationsProcessingMember_019ce26e-a3e5-7543-b0bf-3c9c6b6b219c" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_ResourceSolutionsOperationsMember_019ce26e-a3e5-7b7b-80da-03cbad16ecdd" xlink:href="cwst-20260331.xsd#cwst_ResourceSolutionsOperationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019ce26e-a3e5-74fe-b8fe-de3a817356b6" xlink:to="loc_cwst_ResourceSolutionsOperationsMember_019ce26e-a3e5-7b7b-80da-03cbad16ecdd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_ResourceSolutionsOperationsProcessingMember_019ce26e-a3e5-74bf-9014-5af9005cb764" xlink:href="cwst-20260331.xsd#cwst_ResourceSolutionsOperationsProcessingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cwst_ResourceSolutionsOperationsMember_019ce26e-a3e5-7b7b-80da-03cbad16ecdd" xlink:to="loc_cwst_ResourceSolutionsOperationsProcessingMember_019ce26e-a3e5-74bf-9014-5af9005cb764" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_NationalAccountsMember_019ce26e-a3e5-7293-8149-0f88a307a6fc" xlink:href="cwst-20260331.xsd#cwst_NationalAccountsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cwst_ResourceSolutionsOperationsMember_019ce26e-a3e5-7b7b-80da-03cbad16ecdd" xlink:to="loc_cwst_NationalAccountsMember_019ce26e-a3e5-7293-8149-0f88a307a6fc" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Revenues_019ce26e-a3e5-7a4c-929a-78bf7170e734" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Revenues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_019ce26e-a3e5-7ef7-95ce-d1b02b62289b" xlink:to="loc_us-gaap_Revenues_019ce26e-a3e5-7a4c-929a-78bf7170e734" 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/2025/elts/us-roles-2025.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/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesAbstract"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtAndCapitalLeaseObligations" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.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>
