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

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:019f1393-d4da-7dd6-9bbf-3d29cee7a7cc,g:7ae57e59-b664-4f2b-98d9-1d2ef2028fe9-->
<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://www.energyfocusinc.com/role/CONDENSEDCONSOLIDATEDBALANCESHEETS" xlink:type="simple" xlink:href="efoi-20260630.xsd#CONDENSEDCONSOLIDATEDBALANCESHEETS"/>
  <link:definitionLink xlink:role="http://www.energyfocusinc.com/role/CONDENSEDCONSOLIDATEDBALANCESHEETS" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019f1393-d5ac-7588-90a5-38e0a9724d7b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019f1393-d5ac-7994-bba6-39138c0d5676" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019f1393-d5ac-7588-90a5-38e0a9724d7b" xlink:to="loc_us-gaap_StatementTable_019f1393-d5ac-7994-bba6-39138c0d5676" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019f1393-d5ac-79d4-8b16-297864cfe09d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019f1393-d5ac-7994-bba6-39138c0d5676" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019f1393-d5ac-79d4-8b16-297864cfe09d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_019f1393-d5ac-79d4-8b16-297864cfe09d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019f1393-d5ac-79d4-8b16-297864cfe09d" xlink:to="loc_us-gaap_RelatedPartyDomain_019f1393-d5ac-79d4-8b16-297864cfe09d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_019f1393-d5ac-77d8-bc8a-e39599e8fa2b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019f1393-d5ac-79d4-8b16-297864cfe09d" xlink:to="loc_us-gaap_RelatedPartyDomain_019f1393-d5ac-77d8-bc8a-e39599e8fa2b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NonrelatedPartyMember_019f1393-d5ac-7928-b49a-99c0759dc12c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NonrelatedPartyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_019f1393-d5ac-77d8-bc8a-e39599e8fa2b" xlink:to="loc_us-gaap_NonrelatedPartyMember_019f1393-d5ac-7928-b49a-99c0759dc12c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyMember_019f1393-d5ac-745f-8403-636ead7fe6e3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_019f1393-d5ac-77d8-bc8a-e39599e8fa2b" xlink:to="loc_us-gaap_RelatedPartyMember_019f1393-d5ac-745f-8403-636ead7fe6e3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsAbstract_019f1393-d5ac-7591-ba1f-1792699cf59d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f1393-d5ac-7588-90a5-38e0a9724d7b" xlink:to="loc_us-gaap_AssetsAbstract_019f1393-d5ac-7591-ba1f-1792699cf59d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsCurrentAbstract_019f1393-d5ac-7b7c-9caf-9d7b4a625cbd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetsCurrentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019f1393-d5ac-7591-ba1f-1792699cf59d" xlink:to="loc_us-gaap_AssetsCurrentAbstract_019f1393-d5ac-7b7c-9caf-9d7b4a625cbd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_019f1393-d5ac-72dc-9d57-e2770a88bb54" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_019f1393-d5ac-7b7c-9caf-9d7b4a625cbd" xlink:to="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_019f1393-d5ac-72dc-9d57-e2770a88bb54" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableNetCurrent_019f1393-d5ac-7884-9e54-c58ecbef6556" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccountsReceivableNetCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_019f1393-d5ac-7b7c-9caf-9d7b4a625cbd" xlink:to="loc_us-gaap_AccountsReceivableNetCurrent_019f1393-d5ac-7884-9e54-c58ecbef6556" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InventoryNet_019f1393-d5ac-70f9-a041-9cf4281a79fe" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InventoryNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_019f1393-d5ac-7b7c-9caf-9d7b4a625cbd" xlink:to="loc_us-gaap_InventoryNet_019f1393-d5ac-70f9-a041-9cf4281a79fe" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DepositsAssetsCurrent_019f1393-d5ac-753a-b5ed-3ab301867378" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DepositsAssetsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_019f1393-d5ac-7b7c-9caf-9d7b4a625cbd" xlink:to="loc_us-gaap_DepositsAssetsCurrent_019f1393-d5ac-753a-b5ed-3ab301867378" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrepaidExpenseAndOtherAssetsCurrent_019f1393-d5ac-7c1e-ad50-c4e2f8f90329" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PrepaidExpenseAndOtherAssetsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_019f1393-d5ac-7b7c-9caf-9d7b4a625cbd" xlink:to="loc_us-gaap_PrepaidExpenseAndOtherAssetsCurrent_019f1393-d5ac-7c1e-ad50-c4e2f8f90329" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsCurrent_019f1393-d5ac-77fa-ab1e-77052cc85e76" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_019f1393-d5ac-7b7c-9caf-9d7b4a625cbd" xlink:to="loc_us-gaap_AssetsCurrent_019f1393-d5ac-77fa-ab1e-77052cc85e76" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization_019f1393-d5ac-7b5a-8113-02ab49fe50d8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019f1393-d5ac-7591-ba1f-1792699cf59d" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization_019f1393-d5ac-7b5a-8113-02ab49fe50d8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseRightOfUseAsset_019f1393-d5ac-716b-ae5f-78de3ff6cf77" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingLeaseRightOfUseAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019f1393-d5ac-7591-ba1f-1792699cf59d" xlink:to="loc_us-gaap_OperatingLeaseRightOfUseAsset_019f1393-d5ac-716b-ae5f-78de3ff6cf77" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures_019f1684-0a67-7487-948e-512680344663" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019f1393-d5ac-7591-ba1f-1792699cf59d" xlink:to="loc_us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures_019f1684-0a67-7487-948e-512680344663" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Assets_019f1393-d5ac-77f8-bc65-1bd4d961a82f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Assets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019f1393-d5ac-7591-ba1f-1792699cf59d" xlink:to="loc_us-gaap_Assets_019f1393-d5ac-77f8-bc65-1bd4d961a82f" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAbstract_019f1393-d5ac-75ca-8ce7-4245a90bba4e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LiabilitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f1393-d5ac-7588-90a5-38e0a9724d7b" xlink:to="loc_us-gaap_LiabilitiesAbstract_019f1393-d5ac-75ca-8ce7-4245a90bba4e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesCurrentAbstract_019f1393-d5ac-754b-8247-1836a324ac59" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LiabilitiesCurrentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAbstract_019f1393-d5ac-75ca-8ce7-4245a90bba4e" xlink:to="loc_us-gaap_LiabilitiesCurrentAbstract_019f1393-d5ac-754b-8247-1836a324ac59" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsPayableCurrent_019f1393-d5ac-7a89-8ee8-f924367c3a06" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccountsPayableCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_019f1393-d5ac-754b-8247-1836a324ac59" xlink:to="loc_us-gaap_AccountsPayableCurrent_019f1393-d5ac-7a89-8ee8-f924367c3a06" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherAccruedLiabilitiesCurrent_019f1393-d5ac-719f-b964-059b5158cac0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherAccruedLiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_019f1393-d5ac-754b-8247-1836a324ac59" xlink:to="loc_us-gaap_OtherAccruedLiabilitiesCurrent_019f1393-d5ac-719f-b964-059b5158cac0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccruedProfessionalFeesCurrent_019f1393-d5ac-7e4c-8a64-886f31e2d415" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccruedProfessionalFeesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_019f1393-d5ac-754b-8247-1836a324ac59" xlink:to="loc_us-gaap_AccruedProfessionalFeesCurrent_019f1393-d5ac-7e4c-8a64-886f31e2d415" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccruedEmployeeBenefitsCurrent_019f1393-d5ac-7256-9c3e-9fd6a65a0fbe" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccruedEmployeeBenefitsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_019f1393-d5ac-754b-8247-1836a324ac59" xlink:to="loc_us-gaap_AccruedEmployeeBenefitsCurrent_019f1393-d5ac-7256-9c3e-9fd6a65a0fbe" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccruedSalesCommissionCurrent_019f1393-d5ac-783b-ab42-bd2eb12a1e62" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccruedSalesCommissionCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_019f1393-d5ac-754b-8247-1836a324ac59" xlink:to="loc_us-gaap_AccruedSalesCommissionCurrent_019f1393-d5ac-783b-ab42-bd2eb12a1e62" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StandardProductWarrantyAccrualCurrent_019f1393-d5ac-7fbf-a00e-f92821f1d1b4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StandardProductWarrantyAccrualCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_019f1393-d5ac-754b-8247-1836a324ac59" xlink:to="loc_us-gaap_StandardProductWarrantyAccrualCurrent_019f1393-d5ac-7fbf-a00e-f92821f1d1b4" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseLiabilityCurrent_019f1393-d5ac-7a54-a5cf-b4b35d7195b3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingLeaseLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_019f1393-d5ac-754b-8247-1836a324ac59" xlink:to="loc_us-gaap_OperatingLeaseLiabilityCurrent_019f1393-d5ac-7a54-a5cf-b4b35d7195b3" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermBorrowings_019f5b3e-3ac7-7d5a-badc-05fcf09bb97d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShortTermBorrowings"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_019f1393-d5ac-754b-8247-1836a324ac59" xlink:to="loc_us-gaap_ShortTermBorrowings_019f5b3e-3ac7-7d5a-badc-05fcf09bb97d" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesCurrent_019f1393-d5ac-7213-8225-702f88ef80ad" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_019f1393-d5ac-754b-8247-1836a324ac59" xlink:to="loc_us-gaap_LiabilitiesCurrent_019f1393-d5ac-7213-8225-702f88ef80ad" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseLiabilityNoncurrent_019f1393-d5ac-72cc-a957-d9849081350c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingLeaseLiabilityNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAbstract_019f1393-d5ac-75ca-8ce7-4245a90bba4e" xlink:to="loc_us-gaap_OperatingLeaseLiabilityNoncurrent_019f1393-d5ac-72cc-a957-d9849081350c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Liabilities_019f1393-d5ac-745b-82ce-cd0bfea38521" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Liabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAbstract_019f1393-d5ac-75ca-8ce7-4245a90bba4e" xlink:to="loc_us-gaap_Liabilities_019f1393-d5ac-745b-82ce-cd0bfea38521" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityAbstract_019f1393-d5ac-79f1-ab72-9c63634f4511" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquityAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f1393-d5ac-7588-90a5-38e0a9724d7b" xlink:to="loc_us-gaap_StockholdersEquityAbstract_019f1393-d5ac-79f1-ab72-9c63634f4511" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockValue_019f1393-d5ac-776b-8353-828e99dd5b37" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PreferredStockValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_019f1393-d5ac-79f1-ab72-9c63634f4511" xlink:to="loc_us-gaap_PreferredStockValue_019f1393-d5ac-776b-8353-828e99dd5b37" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockValue_019f1393-d5ac-720b-a23a-cb128340d932" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_019f1393-d5ac-79f1-ab72-9c63634f4511" xlink:to="loc_us-gaap_CommonStockValue_019f1393-d5ac-720b-a23a-cb128340d932" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalCommonStock_019f1393-d5ac-7491-9d6a-ed397c651416" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdditionalPaidInCapitalCommonStock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_019f1393-d5ac-79f1-ab72-9c63634f4511" xlink:to="loc_us-gaap_AdditionalPaidInCapitalCommonStock_019f1393-d5ac-7491-9d6a-ed397c651416" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_019f1393-d5ac-7a7e-b4e9-0e156dee32fe" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_019f1393-d5ac-79f1-ab72-9c63634f4511" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_019f1393-d5ac-7a7e-b4e9-0e156dee32fe" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsAccumulatedDeficit_019f1393-d5ac-79d5-a3d3-e38593b53bb8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RetainedEarningsAccumulatedDeficit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_019f1393-d5ac-79f1-ab72-9c63634f4511" xlink:to="loc_us-gaap_RetainedEarningsAccumulatedDeficit_019f1393-d5ac-79d5-a3d3-e38593b53bb8" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019f1393-d5ac-79f9-8515-153273c19b8c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_019f1393-d5ac-79f1-ab72-9c63634f4511" xlink:to="loc_us-gaap_StockholdersEquity_019f1393-d5ac-79f9-8515-153273c19b8c" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAndStockholdersEquity_019f1393-d5ac-7a7b-946b-a86df37df794" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LiabilitiesAndStockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f1393-d5ac-7588-90a5-38e0a9724d7b" xlink:to="loc_us-gaap_LiabilitiesAndStockholdersEquity_019f1393-d5ac-7a7b-946b-a86df37df794" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.energyfocusinc.com/role/CONDENSEDCONSOLIDATEDBALANCESHEETSParentheticals" xlink:type="simple" xlink:href="efoi-20260630.xsd#CONDENSEDCONSOLIDATEDBALANCESHEETSParentheticals"/>
  <link:definitionLink xlink:role="http://www.energyfocusinc.com/role/CONDENSEDCONSOLIDATEDBALANCESHEETSParentheticals" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019f1393-d5ac-7f2e-9830-dfc24478e773" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019f1393-d5ac-7281-93ec-710f6c0d961f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019f1393-d5ac-7f2e-9830-dfc24478e773" xlink:to="loc_us-gaap_StatementTable_019f1393-d5ac-7281-93ec-710f6c0d961f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_019f1393-d5ac-77da-b3a5-b1994130e542" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019f1393-d5ac-7281-93ec-710f6c0d961f" xlink:to="loc_us-gaap_StatementClassOfStockAxis_019f1393-d5ac-77da-b3a5-b1994130e542" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019f1393-d5ac-77da-b3a5-b1994130e542_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019f1393-d5ac-77da-b3a5-b1994130e542" xlink:to="loc_us-gaap_ClassOfStockDomain_019f1393-d5ac-77da-b3a5-b1994130e542_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019f1393-d5ac-739a-8b7b-932b52b413c2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019f1393-d5ac-77da-b3a5-b1994130e542" xlink:to="loc_us-gaap_ClassOfStockDomain_019f1393-d5ac-739a-8b7b-932b52b413c2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertiblePreferredStockMember_019f1393-d5ac-72a7-aad8-996f475e3710" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConvertiblePreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019f1393-d5ac-739a-8b7b-932b52b413c2" xlink:to="loc_us-gaap_ConvertiblePreferredStockMember_019f1393-d5ac-72a7-aad8-996f475e3710" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent_019f1393-d5ac-7fa2-bba8-2dfacf12105b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f1393-d5ac-7f2e-9830-dfc24478e773" xlink:to="loc_us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent_019f1393-d5ac-7fa2-bba8-2dfacf12105b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockParOrStatedValuePerShare_019f1393-d5ac-7a71-a883-44cd7be94558" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PreferredStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f1393-d5ac-7f2e-9830-dfc24478e773" xlink:to="loc_us-gaap_PreferredStockParOrStatedValuePerShare_019f1393-d5ac-7a71-a883-44cd7be94558" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesAuthorized_019f1393-d5ac-759a-900f-0602e712cba7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PreferredStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f1393-d5ac-7f2e-9830-dfc24478e773" xlink:to="loc_us-gaap_PreferredStockSharesAuthorized_019f1393-d5ac-759a-900f-0602e712cba7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesIssued_019f1393-d5ac-726d-9087-e48ac0ba84ab" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PreferredStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f1393-d5ac-7f2e-9830-dfc24478e773" xlink:to="loc_us-gaap_PreferredStockSharesIssued_019f1393-d5ac-726d-9087-e48ac0ba84ab" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesOutstanding_019f1393-d5ac-7ac6-9de2-58a646d8a21d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PreferredStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f1393-d5ac-7f2e-9830-dfc24478e773" xlink:to="loc_us-gaap_PreferredStockSharesOutstanding_019f1393-d5ac-7ac6-9de2-58a646d8a21d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_019f1393-d5ac-7e82-b95f-50831a89475a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f1393-d5ac-7f2e-9830-dfc24478e773" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_019f1393-d5ac-7e82-b95f-50831a89475a" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesAuthorized_019f1393-d5ac-7428-b5ee-e1caea0275f1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f1393-d5ac-7f2e-9830-dfc24478e773" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_019f1393-d5ac-7428-b5ee-e1caea0275f1" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesIssued_019f1393-d5ac-7076-93b0-ecc6cebe89dd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f1393-d5ac-7f2e-9830-dfc24478e773" xlink:to="loc_us-gaap_CommonStockSharesIssued_019f1393-d5ac-7076-93b0-ecc6cebe89dd" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019f1393-d5ac-72bf-af44-7257eb566bd2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f1393-d5ac-7f2e-9830-dfc24478e773" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_019f1393-d5ac-72bf-af44-7257eb566bd2" xlink:type="arc" order="8"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.energyfocusinc.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCHANGESINSTOCKHOLDERSEQUITY" xlink:type="simple" xlink:href="efoi-20260630.xsd#CONDENSEDCONSOLIDATEDSTATEMENTSOFCHANGESINSTOCKHOLDERSEQUITY"/>
  <link:definitionLink xlink:role="http://www.energyfocusinc.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCHANGESINSTOCKHOLDERSEQUITY" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019f1393-d5ac-72f8-beaf-d857b1273bd2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019f1393-d5ac-79b3-b55d-de00717ccceb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019f1393-d5ac-72f8-beaf-d857b1273bd2" xlink:to="loc_us-gaap_StatementTable_019f1393-d5ac-79b3-b55d-de00717ccceb" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019f1393-d5ac-7f82-8cb9-0d567870e8c1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019f1393-d5ac-79b3-b55d-de00717ccceb" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019f1393-d5ac-7f82-8cb9-0d567870e8c1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019f1393-d5ac-7f82-8cb9-0d567870e8c1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019f1393-d5ac-7f82-8cb9-0d567870e8c1" xlink:to="loc_us-gaap_EquityComponentDomain_019f1393-d5ac-7f82-8cb9-0d567870e8c1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019f1393-d5ac-7f21-a0b0-597df5763aef" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019f1393-d5ac-7f82-8cb9-0d567870e8c1" xlink:to="loc_us-gaap_EquityComponentDomain_019f1393-d5ac-7f21-a0b0-597df5763aef" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockMember_019f1393-d5ac-7add-9052-4e4a2ea1b90a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019f1393-d5ac-7f21-a0b0-597df5763aef" xlink:to="loc_us-gaap_PreferredStockMember_019f1393-d5ac-7add-9052-4e4a2ea1b90a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_019f1393-d5ac-753a-8c2f-670f36de39bd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019f1393-d5ac-7f21-a0b0-597df5763aef" xlink:to="loc_us-gaap_CommonStockMember_019f1393-d5ac-753a-8c2f-670f36de39bd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_019f1393-d5ac-7ca1-ad04-5adc10075b35" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019f1393-d5ac-7f21-a0b0-597df5763aef" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_019f1393-d5ac-7ca1-ad04-5adc10075b35" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019f1393-d5ac-7723-8b95-6ff1fe41685f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019f1393-d5ac-7f21-a0b0-597df5763aef" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019f1393-d5ac-7723-8b95-6ff1fe41685f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_019f1393-d5ac-7f19-b901-67c5d5d9b12a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019f1393-d5ac-7f21-a0b0-597df5763aef" xlink:to="loc_us-gaap_RetainedEarningsMember_019f1393-d5ac-7f19-b901-67c5d5d9b12a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019f1393-d5ac-7448-84c9-51e78cad9c67" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f1393-d5ac-72f8-beaf-d857b1273bd2" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019f1393-d5ac-7448-84c9-51e78cad9c67" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharesOutstanding_019f1393-d5ac-7a55-930b-b52af45c53c7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019f1393-d5ac-7448-84c9-51e78cad9c67" xlink:to="loc_us-gaap_SharesOutstanding_019f1393-d5ac-7a55-930b-b52af45c53c7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019f1393-d5ac-7183-a9a3-8f9315a93372" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019f1393-d5ac-7448-84c9-51e78cad9c67" xlink:to="loc_us-gaap_StockholdersEquity_019f1393-d5ac-7183-a9a3-8f9315a93372" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_019f1393-d5ac-75fa-93a9-9cec3763bcab" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019f1393-d5ac-7448-84c9-51e78cad9c67" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_019f1393-d5ac-75fa-93a9-9cec3763bcab" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan_019f1393-d5ac-76b8-b7fd-ef28ba84e5b1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019f1393-d5ac-7448-84c9-51e78cad9c67" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan_019f1393-d5ac-76b8-b7fd-ef28ba84e5b1" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation_019f169c-02fc-7149-83d5-447f0f06bf07" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019f1393-d5ac-7448-84c9-51e78cad9c67" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation_019f169c-02fc-7149-83d5-447f0f06bf07" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_019f1393-d5ac-7f93-a261-995180c12ecb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019f1393-d5ac-7448-84c9-51e78cad9c67" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_019f1393-d5ac-7f93-a261-995180c12ecb" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019f1393-d5ac-7b69-b019-0237fa98661e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019f1393-d5ac-7448-84c9-51e78cad9c67" xlink:to="loc_us-gaap_NetIncomeLoss_019f1393-d5ac-7b69-b019-0237fa98661e" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharesOutstanding_019f1393-d5ac-7bdc-a1a2-083934e79573" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharesOutstanding"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019f1393-d5ac-7e6b-868f-5c9600307458" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.energyfocusinc.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCASHFLOWS" xlink:type="simple" xlink:href="efoi-20260630.xsd#CONDENSEDCONSOLIDATEDSTATEMENTSOFCASHFLOWS"/>
  <link:definitionLink xlink:role="http://www.energyfocusinc.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCASHFLOWS" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019f1393-d5ac-7fe6-b85e-ccd55aed394a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019f1393-d5ac-70a6-9cc0-5c71cab88284" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019f1393-d5ac-7fe6-b85e-ccd55aed394a" xlink:to="loc_us-gaap_StatementTable_019f1393-d5ac-70a6-9cc0-5c71cab88284" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019f1393-d5ac-7592-a987-eaf6c5990b1d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019f1393-d5ac-70a6-9cc0-5c71cab88284" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019f1393-d5ac-7592-a987-eaf6c5990b1d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_019f1393-d5ac-7592-a987-eaf6c5990b1d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019f1393-d5ac-7592-a987-eaf6c5990b1d" xlink:to="loc_us-gaap_RelatedPartyDomain_019f1393-d5ac-7592-a987-eaf6c5990b1d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_019f1393-d5ac-7809-83eb-2b9af7380197" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019f1393-d5ac-7592-a987-eaf6c5990b1d" xlink:to="loc_us-gaap_RelatedPartyDomain_019f1393-d5ac-7809-83eb-2b9af7380197" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NonrelatedPartyMember_019f1393-d5ac-7620-9746-863180a05590" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NonrelatedPartyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_019f1393-d5ac-7809-83eb-2b9af7380197" xlink:to="loc_us-gaap_NonrelatedPartyMember_019f1393-d5ac-7620-9746-863180a05590" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyMember_019f1393-d5ac-7829-b7e4-deb37099ec4d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_019f1393-d5ac-7809-83eb-2b9af7380197" xlink:to="loc_us-gaap_RelatedPartyMember_019f1393-d5ac-7829-b7e4-deb37099ec4d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019f1393-d5ac-7ae8-bbcd-abd9ca11fe6c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f1393-d5ac-7fe6-b85e-ccd55aed394a" xlink:to="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019f1393-d5ac-7ae8-bbcd-abd9ca11fe6c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019f1393-d5ac-7cf2-892c-f8ab11eb00b3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019f1393-d5ac-7ae8-bbcd-abd9ca11fe6c" xlink:to="loc_us-gaap_NetIncomeLoss_019f1393-d5ac-7cf2-892c-f8ab11eb00b3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019f1393-d5ac-7378-b079-18b7ec7afee0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019f1393-d5ac-7ae8-bbcd-abd9ca11fe6c" xlink:to="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019f1393-d5ac-7378-b079-18b7ec7afee0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignCurrencyTransactionGainLossAfterTax_019f1393-d5ac-7602-8ed4-f8a2562277c8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ForeignCurrencyTransactionGainLossAfterTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019f1393-d5ac-7378-b079-18b7ec7afee0" xlink:to="loc_us-gaap_ForeignCurrencyTransactionGainLossAfterTax_019f1393-d5ac-7602-8ed4-f8a2562277c8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_GainLossOnSettlementOfVendorObligations_019f1393-d5ac-79e2-85a7-7889d0ffb148" xlink:href="efoi-20260630.xsd#efoi_GainLossOnSettlementOfVendorObligations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019f1393-d5ac-7378-b079-18b7ec7afee0" xlink:to="loc_efoi_GainLossOnSettlementOfVendorObligations_019f1393-d5ac-79e2-85a7-7889d0ffb148" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Depreciation_019f1393-d5ac-7b97-bcc3-d6ea788f305a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Depreciation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019f1393-d5ac-7378-b079-18b7ec7afee0" xlink:to="loc_us-gaap_Depreciation_019f1393-d5ac-7b97-bcc3-d6ea788f305a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensation_019f1393-d5ac-7124-a09c-0e1cec0d4639" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019f1393-d5ac-7378-b079-18b7ec7afee0" xlink:to="loc_us-gaap_ShareBasedCompensation_019f1393-d5ac-7124-a09c-0e1cec0d4639" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProvisionForDoubtfulAccounts_019f1393-d5ac-7d27-9dce-82a3e42f0a02" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProvisionForDoubtfulAccounts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019f1393-d5ac-7378-b079-18b7ec7afee0" xlink:to="loc_us-gaap_ProvisionForDoubtfulAccounts_019f1393-d5ac-7d27-9dce-82a3e42f0a02" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InventoryWriteDown_019f1393-d5ac-7dca-b95c-5ba74b453068" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InventoryWriteDown"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019f1393-d5ac-7378-b079-18b7ec7afee0" xlink:to="loc_us-gaap_InventoryWriteDown_019f1393-d5ac-7dca-b95c-5ba74b453068" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_IncreaseDecreaseInProvisionForWarranties_019f1393-d5ac-7f2e-9d79-95b60da8594e" xlink:href="efoi-20260630.xsd#efoi_IncreaseDecreaseInProvisionForWarranties"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019f1393-d5ac-7378-b079-18b7ec7afee0" xlink:to="loc_efoi_IncreaseDecreaseInProvisionForWarranties_019f1393-d5ac-7f2e-9d79-95b60da8594e" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_019f1393-d5ac-777e-bea8-d6d9e2d7240e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInOperatingCapitalAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019f1393-d5ac-7ae8-bbcd-abd9ca11fe6c" xlink:to="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_019f1393-d5ac-777e-bea8-d6d9e2d7240e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInAccountsReceivable_019f1393-d5ac-735d-9f41-cbff544529a9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInAccountsReceivable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_019f1393-d5ac-777e-bea8-d6d9e2d7240e" xlink:to="loc_us-gaap_IncreaseDecreaseInAccountsReceivable_019f1393-d5ac-735d-9f41-cbff544529a9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInInventories_019f1393-d5ac-77e2-801d-9eafb3316a96" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInInventories"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_019f1393-d5ac-777e-bea8-d6d9e2d7240e" xlink:to="loc_us-gaap_IncreaseDecreaseInInventories_019f1393-d5ac-77e2-801d-9eafb3316a96" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_IncreaseDecreaseInVendorPrepayments_019f1393-d5ac-7c8f-913c-958f71f72967" xlink:href="efoi-20260630.xsd#efoi_IncreaseDecreaseInVendorPrepayments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_019f1393-d5ac-777e-bea8-d6d9e2d7240e" xlink:to="loc_efoi_IncreaseDecreaseInVendorPrepayments_019f1393-d5ac-7c8f-913c-958f71f72967" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets_019f1393-d5ac-7a87-b680-f6a9f6fe8d51" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_019f1393-d5ac-777e-bea8-d6d9e2d7240e" xlink:to="loc_us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets_019f1393-d5ac-7a87-b680-f6a9f6fe8d51" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInAccountsPayable_019f1393-d5ac-777a-8150-c1da483d87b5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInAccountsPayable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_019f1393-d5ac-777e-bea8-d6d9e2d7240e" xlink:to="loc_us-gaap_IncreaseDecreaseInAccountsPayable_019f1393-d5ac-777a-8150-c1da483d87b5" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInAccruedLiabilities_019f1393-d5ac-74e6-b4b0-78e1ea207b61" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInAccruedLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_019f1393-d5ac-777e-bea8-d6d9e2d7240e" xlink:to="loc_us-gaap_IncreaseDecreaseInAccruedLiabilities_019f1393-d5ac-74e6-b4b0-78e1ea207b61" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInOtherOperatingCapitalNet_019f1393-d5ac-7238-978e-a096cc584dcb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInOtherOperatingCapitalNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_019f1393-d5ac-777e-bea8-d6d9e2d7240e" xlink:to="loc_us-gaap_IncreaseDecreaseInOtherOperatingCapitalNet_019f1393-d5ac-7238-978e-a096cc584dcb" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivities_019f1393-d5ac-7aa5-b630-a0d16f162d6c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_019f1393-d5ac-777e-bea8-d6d9e2d7240e" xlink:to="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivities_019f1393-d5ac-7aa5-b630-a0d16f162d6c" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_019f1393-d5ac-7546-87d9-943c8893c5f3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019f1393-d5ac-7ae8-bbcd-abd9ca11fe6c" xlink:to="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_019f1393-d5ac-7546-87d9-943c8893c5f3" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperationsAbstract_019f1393-d5ac-704b-bda7-1d1680ec25a0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperationsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f1393-d5ac-7fe6-b85e-ccd55aed394a" xlink:to="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperationsAbstract_019f1393-d5ac-704b-bda7-1d1680ec25a0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquirePropertyPlantAndEquipment_019f1393-d5ac-7b2c-9710-43a9299713e1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PaymentsToAcquirePropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperationsAbstract_019f1393-d5ac-704b-bda7-1d1680ec25a0" xlink:to="loc_us-gaap_PaymentsToAcquirePropertyPlantAndEquipment_019f1393-d5ac-7b2c-9710-43a9299713e1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireInterestInJointVenture_019f5b38-44bf-771b-b144-83ea1ad8c22d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PaymentsToAcquireInterestInJointVenture"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperationsAbstract_019f1393-d5ac-704b-bda7-1d1680ec25a0" xlink:to="loc_us-gaap_PaymentsToAcquireInterestInJointVenture_019f5b38-44bf-771b-b144-83ea1ad8c22d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInInvestingActivities_019f1393-d5ac-73a8-a61b-70e0ebefb9c3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperationsAbstract_019f1393-d5ac-704b-bda7-1d1680ec25a0" xlink:to="loc_us-gaap_NetCashProvidedByUsedInInvestingActivities_019f1393-d5ac-73a8-a61b-70e0ebefb9c3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019f1393-d5ac-73f1-8813-6e6b503d80ed" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f1393-d5ac-7fe6-b85e-ccd55aed394a" xlink:to="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019f1393-d5ac-73f1-8813-6e6b503d80ed" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfCommonStock_019f1393-d5ac-7950-906a-21d9d2d67ed0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromIssuanceOfCommonStock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019f1393-d5ac-73f1-8813-6e6b503d80ed" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfCommonStock_019f1393-d5ac-7950-906a-21d9d2d67ed0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromShortTermDebt_019fa769-ed0a-7594-81be-61941cf39d66" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromShortTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019f1393-d5ac-73f1-8813-6e6b503d80ed" xlink:to="loc_us-gaap_ProceedsFromShortTermDebt_019fa769-ed0a-7594-81be-61941cf39d66" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInFinancingActivities_019f1393-d5ac-7811-b9b8-7f049b6f9ca3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019f1393-d5ac-73f1-8813-6e6b503d80ed" xlink:to="loc_us-gaap_NetCashProvidedByUsedInFinancingActivities_019f1393-d5ac-7811-b9b8-7f049b6f9ca3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectOfExchangeRateOnCashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsIncludingDisposalGroupAndDiscontinuedOperations_019f1393-d5ac-7c5d-8675-0862e7e39b07" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EffectOfExchangeRateOnCashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsIncludingDisposalGroupAndDiscontinuedOperations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f1393-d5ac-7fe6-b85e-ccd55aed394a" xlink:to="loc_us-gaap_EffectOfExchangeRateOnCashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsIncludingDisposalGroupAndDiscontinuedOperations_019f1393-d5ac-7c5d-8675-0862e7e39b07" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect_019f1393-d5ac-73ed-a66d-7957de68bc81" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f1393-d5ac-7fe6-b85e-ccd55aed394a" xlink:to="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect_019f1393-d5ac-73ed-a66d-7957de68bc81" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_019f1393-d5ac-7825-aadf-72c6649334fe" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f1393-d5ac-7fe6-b85e-ccd55aed394a" xlink:to="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_019f1393-d5ac-7825-aadf-72c6649334fe" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_019f1393-d5ac-7deb-9364-564880d09841" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashFlowNoncashInvestingAndFinancingActivitiesDisclosureAbstract_019f1393-d5ac-7fa3-a592-8616c36da020" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashFlowNoncashInvestingAndFinancingActivitiesDisclosureAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f1393-d5ac-7fe6-b85e-ccd55aed394a" xlink:to="loc_us-gaap_CashFlowNoncashInvestingAndFinancingActivitiesDisclosureAbstract_019f1393-d5ac-7fa3-a592-8616c36da020" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CapitalExpendituresIncurredButNotYetPaid_019f1393-d5ac-790f-8f69-03cf52946268" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CapitalExpendituresIncurredButNotYetPaid"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashFlowNoncashInvestingAndFinancingActivitiesDisclosureAbstract_019f1393-d5ac-7fa3-a592-8616c36da020" xlink:to="loc_us-gaap_CapitalExpendituresIncurredButNotYetPaid_019f1393-d5ac-790f-8f69-03cf52946268" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.energyfocusinc.com/role/BASISOFPRESENTATIONANDSUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESBasisofPresentationDetails" xlink:type="simple" xlink:href="efoi-20260630.xsd#BASISOFPRESENTATIONANDSUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESBasisofPresentationDetails"/>
  <link:definitionLink xlink:role="http://www.energyfocusinc.com/role/BASISOFPRESENTATIONANDSUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESBasisofPresentationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_019f16d2-f4f7-794b-b34b-25ed0e997bb6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_019f1393-d5ac-7adc-a5c3-19ea3df25cf6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_019f16d2-f4f7-794b-b34b-25ed0e997bb6" xlink:to="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_019f1393-d5ac-7adc-a5c3-19ea3df25cf6" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_019f16d2-f4f7-72d3-8a39-862321758f40" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_019f1393-d5ac-7adc-a5c3-19ea3df25cf6" xlink:to="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_019f16d2-f4f7-72d3-8a39-862321758f40" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_EquityMethodInvesteeNameDomain_019f16d2-f4f7-72d3-8a39-862321758f40_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_EquityMethodInvesteeNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_019f16d2-f4f7-72d3-8a39-862321758f40" xlink:to="loc_srt_EquityMethodInvesteeNameDomain_019f16d2-f4f7-72d3-8a39-862321758f40_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_EquityMethodInvesteeNameDomain_019f16d2-f4f7-7b94-ab60-e48bec0aa360" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_EquityMethodInvesteeNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_019f16d2-f4f7-72d3-8a39-862321758f40" xlink:to="loc_srt_EquityMethodInvesteeNameDomain_019f16d2-f4f7-7b94-ab60-e48bec0aa360" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_AsakuraEnergyStoragePowerPlantMember_019f16d2-f4f7-7109-91a8-96e7186bfd1e" xlink:href="efoi-20260630.xsd#efoi_AsakuraEnergyStoragePowerPlantMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_EquityMethodInvesteeNameDomain_019f16d2-f4f7-7b94-ab60-e48bec0aa360" xlink:to="loc_efoi_AsakuraEnergyStoragePowerPlantMember_019f16d2-f4f7-7109-91a8-96e7186bfd1e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_019fb90d-ecb5-7c0b-a18f-f54a6bf7777b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_019f1393-d5ac-7adc-a5c3-19ea3df25cf6" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_019fb90d-ecb5-7c0b-a18f-f54a6bf7777b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_019fb90d-ecb5-7c0b-a18f-f54a6bf7777b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_019fb90d-ecb5-7c0b-a18f-f54a6bf7777b" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_019fb90d-ecb5-7c0b-a18f-f54a6bf7777b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_019fb90d-ecb5-7929-b144-ef840d96f7d5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_019fb90d-ecb5-7c0b-a18f-f54a6bf7777b" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_019fb90d-ecb5-7929-b144-ef840d96f7d5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventMember_019fb90d-ecb5-70ab-a8b1-79f433e79e55" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_019fb90d-ecb5-7929-b144-ef840d96f7d5" xlink:to="loc_us-gaap_SubsequentEventMember_019fb90d-ecb5-70ab-a8b1-79f433e79e55" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019f16d2-f4f7-72a7-b16f-a64bedaf51e2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_019f16d2-f4f7-794b-b34b-25ed0e997bb6" xlink:to="loc_us-gaap_NetIncomeLoss_019f16d2-f4f7-72a7-b16f-a64bedaf51e2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_019f16d2-f4f7-7fcb-b72b-958d47fd2741" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_019f16d2-f4f7-794b-b34b-25ed0e997bb6" xlink:to="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_019f16d2-f4f7-7fcb-b72b-958d47fd2741" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityMethodInvestmentOwnershipPercentage_019f16d2-f4f7-7410-8514-1bffb7d7ec4c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityMethodInvestmentOwnershipPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_019f16d2-f4f7-794b-b34b-25ed0e997bb6" xlink:to="loc_us-gaap_EquityMethodInvestmentOwnershipPercentage_019f16d2-f4f7-7410-8514-1bffb7d7ec4c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityMethodInvestments_019f16d2-f4f7-736f-ad83-cf1275684c54" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityMethodInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_019f16d2-f4f7-794b-b34b-25ed0e997bb6" xlink:to="loc_us-gaap_EquityMethodInvestments_019f16d2-f4f7-736f-ad83-cf1275684c54" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures_019f5b51-bd80-7584-bb60-4a0dbd80a70c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_019f16d2-f4f7-794b-b34b-25ed0e997bb6" xlink:to="loc_us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures_019f5b51-bd80-7584-bb60-4a0dbd80a70c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_EquityMethodInvestmentRemainingBalanceExpectedToBeFunded_019f16d2-f4f7-786f-957c-a16996e977f0" xlink:href="efoi-20260630.xsd#efoi_EquityMethodInvestmentRemainingBalanceExpectedToBeFunded"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_019f16d2-f4f7-794b-b34b-25ed0e997bb6" xlink:to="loc_efoi_EquityMethodInvestmentRemainingBalanceExpectedToBeFunded_019f16d2-f4f7-786f-957c-a16996e977f0" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.energyfocusinc.com/role/BASISOFPRESENTATIONANDSUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESScheduleofDisaggregationofProductandServiceNetSalesDetails" xlink:type="simple" xlink:href="efoi-20260630.xsd#BASISOFPRESENTATIONANDSUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESScheduleofDisaggregationofProductandServiceNetSalesDetails"/>
  <link:definitionLink xlink:role="http://www.energyfocusinc.com/role/BASISOFPRESENTATIONANDSUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESScheduleofDisaggregationofProductandServiceNetSalesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_019f1393-d5ac-79ea-9051-7d0a75bf41b1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_019f1393-d5ac-73c6-9f07-4d37bf37bc78" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_019f1393-d5ac-79ea-9051-7d0a75bf41b1" xlink:to="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_019f1393-d5ac-73c6-9f07-4d37bf37bc78" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_019f1393-d5ac-7fb6-8669-c2492d19d5a7" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_019f1393-d5ac-73c6-9f07-4d37bf37bc78" xlink:to="loc_srt_ProductOrServiceAxis_019f1393-d5ac-7fb6-8669-c2492d19d5a7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019f1393-d5ac-7fb6-8669-c2492d19d5a7_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_019f1393-d5ac-7fb6-8669-c2492d19d5a7" xlink:to="loc_srt_ProductsAndServicesDomain_019f1393-d5ac-7fb6-8669-c2492d19d5a7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019f1393-d5ac-7331-bee1-5a54c682590a" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_019f1393-d5ac-7fb6-8669-c2492d19d5a7" xlink:to="loc_srt_ProductsAndServicesDomain_019f1393-d5ac-7331-bee1-5a54c682590a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_CommercialProductsMember_019f1393-d5ac-7de9-b5c4-992256040d0c" xlink:href="efoi-20260630.xsd#efoi_CommercialProductsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019f1393-d5ac-7331-bee1-5a54c682590a" xlink:to="loc_efoi_CommercialProductsMember_019f1393-d5ac-7de9-b5c4-992256040d0c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_MMMProductsMember_019f1393-d5ac-772c-9fea-146ca955951b" xlink:href="efoi-20260630.xsd#efoi_MMMProductsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019f1393-d5ac-7331-bee1-5a54c682590a" xlink:to="loc_efoi_MMMProductsMember_019f1393-d5ac-772c-9fea-146ca955951b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_SetupServiceMember_019f1393-d5ac-7f2e-b4d3-8957470575c3" xlink:href="efoi-20260630.xsd#efoi_SetupServiceMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019f1393-d5ac-7331-bee1-5a54c682590a" xlink:to="loc_efoi_SetupServiceMember_019f1393-d5ac-7f2e-b4d3-8957470575c3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Revenues_019f1393-d5ac-7eea-9b22-e82b88f2b348" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Revenues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_019f1393-d5ac-79ea-9051-7d0a75bf41b1" xlink:to="loc_us-gaap_Revenues_019f1393-d5ac-7eea-9b22-e82b88f2b348" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.energyfocusinc.com/role/BASISOFPRESENTATIONANDSUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESAccountsReceivableDetails" xlink:type="simple" xlink:href="efoi-20260630.xsd#BASISOFPRESENTATIONANDSUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESAccountsReceivableDetails"/>
  <link:definitionLink xlink:role="http://www.energyfocusinc.com/role/BASISOFPRESENTATIONANDSUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESAccountsReceivableDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskLineItems_019f1393-d5ac-71d6-af11-15b08881bae5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTable_019f1393-d5ac-7582-9369-9cf320853e0f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_019f1393-d5ac-71d6-af11-15b08881bae5" xlink:to="loc_us-gaap_ConcentrationRiskTable_019f1393-d5ac-7582-9369-9cf320853e0f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019f16e0-8459-7ceb-80cb-16283edd7c04" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_019f1393-d5ac-7582-9369-9cf320853e0f" xlink:to="loc_srt_RangeAxis_019f16e0-8459-7ceb-80cb-16283edd7c04" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019f16e0-8459-7ceb-80cb-16283edd7c04_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019f16e0-8459-7ceb-80cb-16283edd7c04" xlink:to="loc_srt_RangeMember_019f16e0-8459-7ceb-80cb-16283edd7c04_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019f16e0-8459-7748-a58a-5c81a74587a6" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019f16e0-8459-7ceb-80cb-16283edd7c04" xlink:to="loc_srt_RangeMember_019f16e0-8459-7748-a58a-5c81a74587a6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019f16e0-8459-7406-a7ab-f7840bcd2eec" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019f16e0-8459-7748-a58a-5c81a74587a6" xlink:to="loc_srt_MinimumMember_019f16e0-8459-7406-a7ab-f7840bcd2eec" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019f16e0-8459-7250-9f57-62e42d43637a" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019f16e0-8459-7748-a58a-5c81a74587a6" xlink:to="loc_srt_MaximumMember_019f16e0-8459-7250-9f57-62e42d43637a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_ContractWithCustomerPaymentTerms_019f1393-d5ac-708e-9b25-eb9fabfb7101" xlink:href="efoi-20260630.xsd#efoi_ContractWithCustomerPaymentTerms"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_019f1393-d5ac-71d6-af11-15b08881bae5" xlink:to="loc_efoi_ContractWithCustomerPaymentTerms_019f1393-d5ac-708e-9b25-eb9fabfb7101" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.energyfocusinc.com/role/BASISOFPRESENTATIONANDSUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESScheduleofNetAccountsReceivableDetails" xlink:type="simple" xlink:href="efoi-20260630.xsd#BASISOFPRESENTATIONANDSUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESScheduleofNetAccountsReceivableDetails"/>
  <link:definitionLink xlink:role="http://www.energyfocusinc.com/role/BASISOFPRESENTATIONANDSUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESScheduleofNetAccountsReceivableDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionLineItems_019f1393-d5ac-7ba6-a0d9-0a553055e87e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyTransactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019f1393-d5ac-70bf-9f72-ea6e23be1f48" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019f1393-d5ac-7ba6-a0d9-0a553055e87e" xlink:to="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019f1393-d5ac-70bf-9f72-ea6e23be1f48" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019f1393-d5ac-72b7-9858-c55d25ebe10d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019f1393-d5ac-70bf-9f72-ea6e23be1f48" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019f1393-d5ac-72b7-9858-c55d25ebe10d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_019f1393-d5ac-72b7-9858-c55d25ebe10d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019f1393-d5ac-72b7-9858-c55d25ebe10d" xlink:to="loc_us-gaap_RelatedPartyDomain_019f1393-d5ac-72b7-9858-c55d25ebe10d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_019f1393-d5ac-7419-96d6-133979c30cfc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019f1393-d5ac-72b7-9858-c55d25ebe10d" xlink:to="loc_us-gaap_RelatedPartyDomain_019f1393-d5ac-7419-96d6-133979c30cfc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NonrelatedPartyMember_019f1393-d5ac-750e-a31f-ca51f8d5e0bf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NonrelatedPartyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_019f1393-d5ac-7419-96d6-133979c30cfc" xlink:to="loc_us-gaap_NonrelatedPartyMember_019f1393-d5ac-750e-a31f-ca51f8d5e0bf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableGrossCurrent_019f1393-d5ac-75ef-9ecb-4736db802a20" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccountsReceivableGrossCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019f1393-d5ac-7ba6-a0d9-0a553055e87e" xlink:to="loc_us-gaap_AccountsReceivableGrossCurrent_019f1393-d5ac-75ef-9ecb-4736db802a20" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent_019f1393-d5ac-7fad-9d29-778be9ea3425" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019f1393-d5ac-7ba6-a0d9-0a553055e87e" xlink:to="loc_us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent_019f1393-d5ac-7fad-9d29-778be9ea3425" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableNetCurrent_019f1393-d5ac-7cc4-be14-fff2bd3d9db7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccountsReceivableNetCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019f1393-d5ac-7ba6-a0d9-0a553055e87e" xlink:to="loc_us-gaap_AccountsReceivableNetCurrent_019f1393-d5ac-7cc4-be14-fff2bd3d9db7" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.energyfocusinc.com/role/BASISOFPRESENTATIONANDSUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESScheduleofAllowanceForCreditLossesDetails" xlink:type="simple" xlink:href="efoi-20260630.xsd#BASISOFPRESENTATIONANDSUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESScheduleofAllowanceForCreditLossesDetails"/>
  <link:definitionLink xlink:role="http://www.energyfocusinc.com/role/BASISOFPRESENTATIONANDSUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESScheduleofAllowanceForCreditLossesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionLineItems_019f1393-d5ac-7454-b6a1-1aa79135f46b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyTransactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019f1393-d5ac-7ed5-8204-71468add7e95" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019f1393-d5ac-7454-b6a1-1aa79135f46b" xlink:to="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019f1393-d5ac-7ed5-8204-71468add7e95" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019f1393-d5ac-750e-919d-b743257acba9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019f1393-d5ac-7ed5-8204-71468add7e95" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019f1393-d5ac-750e-919d-b743257acba9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_019f1393-d5ac-750e-919d-b743257acba9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019f1393-d5ac-750e-919d-b743257acba9" xlink:to="loc_us-gaap_RelatedPartyDomain_019f1393-d5ac-750e-919d-b743257acba9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_019f1393-d5ac-7e5d-9d02-7947c3d3dfb1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019f1393-d5ac-750e-919d-b743257acba9" xlink:to="loc_us-gaap_RelatedPartyDomain_019f1393-d5ac-7e5d-9d02-7947c3d3dfb1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NonrelatedPartyMember_019f1393-d5ac-7078-ba42-7f3e84f8e555" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NonrelatedPartyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_019f1393-d5ac-7e5d-9d02-7947c3d3dfb1" xlink:to="loc_us-gaap_NonrelatedPartyMember_019f1393-d5ac-7078-ba42-7f3e84f8e555" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllowanceForDoubtfulAccountsReceivableRollforward_019f1393-d5ac-755c-a06a-de160ca280d5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AllowanceForDoubtfulAccountsReceivableRollforward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019f1393-d5ac-7454-b6a1-1aa79135f46b" xlink:to="loc_us-gaap_AllowanceForDoubtfulAccountsReceivableRollforward_019f1393-d5ac-755c-a06a-de160ca280d5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent_019f1393-d5ac-7c9c-b3fb-7fdc22f07a43" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AllowanceForDoubtfulAccountsReceivableRollforward_019f1393-d5ac-755c-a06a-de160ca280d5" xlink:to="loc_us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent_019f1393-d5ac-7c9c-b3fb-7fdc22f07a43" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_AccountsReceivableAllowanceForCreditLossReserve_019f1393-d5ac-711a-ba49-9f281e875a5d" xlink:href="efoi-20260630.xsd#efoi_AccountsReceivableAllowanceForCreditLossReserve"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AllowanceForDoubtfulAccountsReceivableRollforward_019f1393-d5ac-755c-a06a-de160ca280d5" xlink:to="loc_efoi_AccountsReceivableAllowanceForCreditLossReserve_019f1393-d5ac-711a-ba49-9f281e875a5d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent_019f1393-d5ac-7f21-97e3-dd52d1bf45be" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.energyfocusinc.com/role/BASISOFPRESENTATIONANDSUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESScheduleofNetSalesbyCountryDetails" xlink:type="simple" xlink:href="efoi-20260630.xsd#BASISOFPRESENTATIONANDSUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESScheduleofNetSalesbyCountryDetails"/>
  <link:definitionLink xlink:role="http://www.energyfocusinc.com/role/BASISOFPRESENTATIONANDSUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESScheduleofNetSalesbyCountryDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskLineItems_019fa78d-973a-75c1-a23c-3f62f9bd0b7a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTable_019fa78d-973a-7060-bd30-e448c911eba0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_019fa78d-973a-75c1-a23c-3f62f9bd0b7a" xlink:to="loc_us-gaap_ConcentrationRiskTable_019fa78d-973a-7060-bd30-e448c911eba0" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_019fa78d-973a-748a-bb74-07b001732d9b" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_019fa78d-973a-7060-bd30-e448c911eba0" xlink:to="loc_srt_StatementGeographicalAxis_019fa78d-973a-748a-bb74-07b001732d9b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_019fa78d-973a-748a-bb74-07b001732d9b_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_019fa78d-973a-748a-bb74-07b001732d9b" xlink:to="loc_srt_SegmentGeographicalDomain_019fa78d-973a-748a-bb74-07b001732d9b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_019fa78d-973a-7ed6-a41f-8eb515ddfb3d" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_019fa78d-973a-748a-bb74-07b001732d9b" xlink:to="loc_srt_SegmentGeographicalDomain_019fa78d-973a-7ed6-a41f-8eb515ddfb3d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_019fa78d-973a-7b8e-8291-9eac462f26d2" xlink:href="https://xbrl.sec.gov/country/2026/country-2026.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019fa78d-973a-7ed6-a41f-8eb515ddfb3d" xlink:to="loc_country_US_019fa78d-973a-7b8e-8291-9eac462f26d2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_AU_019fa78d-973a-7553-992b-e753c33beede" xlink:href="https://xbrl.sec.gov/country/2026/country-2026.xsd#country_AU"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019fa78d-973a-7ed6-a41f-8eb515ddfb3d" xlink:to="loc_country_AU_019fa78d-973a-7553-992b-e753c33beede" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_TW_019fa78d-973a-7cb2-add4-19b858b15669" xlink:href="https://xbrl.sec.gov/country/2026/country-2026.xsd#country_TW"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019fa78d-973a-7ed6-a41f-8eb515ddfb3d" xlink:to="loc_country_TW_019fa78d-973a-7cb2-add4-19b858b15669" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_OtherMember_019fa78d-973a-7951-a68e-e9c7641a8646" xlink:href="efoi-20260630.xsd#efoi_OtherMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019fa78d-973a-7ed6-a41f-8eb515ddfb3d" xlink:to="loc_efoi_OtherMember_019fa78d-973a-7951-a68e-e9c7641a8646" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Revenues_019fb9c8-328f-71d8-a381-24fd286009a4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Revenues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_019fa78d-973a-75c1-a23c-3f62f9bd0b7a" xlink:to="loc_us-gaap_Revenues_019fb9c8-328f-71d8-a381-24fd286009a4" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.energyfocusinc.com/role/BASISOFPRESENTATIONANDSUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESNetLossperShareDetails" xlink:type="simple" xlink:href="efoi-20260630.xsd#BASISOFPRESENTATIONANDSUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESNetLossperShareDetails"/>
  <link:definitionLink xlink:role="http://www.energyfocusinc.com/role/BASISOFPRESENTATIONANDSUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESNetLossperShareDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ImpairmentEffectsOnEarningsPerShareLineItems_019f1393-d680-72d6-8639-4cb286ddf857" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ImpairmentEffectsOnEarningsPerShareLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ImpairmentEffectsOnEarningsPerShareTable_019f1393-d680-78f1-8c94-1e9131559857" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ImpairmentEffectsOnEarningsPerShareTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ImpairmentEffectsOnEarningsPerShareLineItems_019f1393-d680-72d6-8639-4cb286ddf857" xlink:to="loc_us-gaap_ImpairmentEffectsOnEarningsPerShareTable_019f1393-d680-78f1-8c94-1e9131559857" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019f1393-d680-7036-b64e-90e62737aae3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ImpairmentEffectsOnEarningsPerShareTable_019f1393-d680-78f1-8c94-1e9131559857" xlink:to="loc_us-gaap_AwardTypeAxis_019f1393-d680-7036-b64e-90e62737aae3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019f1393-d680-7036-b64e-90e62737aae3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019f1393-d680-7036-b64e-90e62737aae3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019f1393-d680-7036-b64e-90e62737aae3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019f1393-d680-7e56-9a98-3bc27ff62546" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019f1393-d680-7036-b64e-90e62737aae3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019f1393-d680-7e56-9a98-3bc27ff62546" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertiblePreferredStockMember_019f1393-d680-7163-bcb9-79f7e6ea68a5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConvertiblePreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019f1393-d680-7e56-9a98-3bc27ff62546" xlink:to="loc_us-gaap_ConvertiblePreferredStockMember_019f1393-d680-7163-bcb9-79f7e6ea68a5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_019f1393-d680-7025-84c4-d83d5173764d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ImpairmentEffectsOnEarningsPerShareLineItems_019f1393-d680-72d6-8639-4cb286ddf857" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_019f1393-d680-7025-84c4-d83d5173764d" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.energyfocusinc.com/role/BASISOFPRESENTATIONANDSUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESProductWarrantiesDetails" xlink:type="simple" xlink:href="efoi-20260630.xsd#BASISOFPRESENTATIONANDSUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESProductWarrantiesDetails"/>
  <link:definitionLink xlink:role="http://www.energyfocusinc.com/role/BASISOFPRESENTATIONANDSUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESProductWarrantiesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProductWarrantyLiabilityLineItems_019f1393-d680-784a-9ab2-5bfe28ef2101" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProductWarrantyLiabilityLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProductWarrantyLiabilityTable_019f1393-d680-7cd6-b257-ce7248325463" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProductWarrantyLiabilityTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ProductWarrantyLiabilityLineItems_019f1393-d680-784a-9ab2-5bfe28ef2101" xlink:to="loc_us-gaap_ProductWarrantyLiabilityTable_019f1393-d680-7cd6-b257-ce7248325463" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_019f1393-d680-75ae-9317-797c08e2200c" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ProductWarrantyLiabilityTable_019f1393-d680-7cd6-b257-ce7248325463" xlink:to="loc_srt_ProductOrServiceAxis_019f1393-d680-75ae-9317-797c08e2200c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019f1393-d680-75ae-9317-797c08e2200c_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_019f1393-d680-75ae-9317-797c08e2200c" xlink:to="loc_srt_ProductsAndServicesDomain_019f1393-d680-75ae-9317-797c08e2200c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019f1393-d680-7fd7-8c9a-192aae6bebe7" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_019f1393-d680-75ae-9317-797c08e2200c" xlink:to="loc_srt_ProductsAndServicesDomain_019f1393-d680-7fd7-8c9a-192aae6bebe7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_CommercialProductsMember_019f1393-d680-7064-a603-adc212b691a9" xlink:href="efoi-20260630.xsd#efoi_CommercialProductsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019f1393-d680-7fd7-8c9a-192aae6bebe7" xlink:to="loc_efoi_CommercialProductsMember_019f1393-d680-7064-a603-adc212b691a9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_MMMLEDProductsMember_019f1393-d680-7051-a0c9-d1dc30c8c0dc" xlink:href="efoi-20260630.xsd#efoi_MMMLEDProductsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019f1393-d680-7fd7-8c9a-192aae6bebe7" xlink:to="loc_efoi_MMMLEDProductsMember_019f1393-d680-7051-a0c9-d1dc30c8c0dc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019f1393-d680-7f83-adac-fd79af324b09" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ProductWarrantyLiabilityTable_019f1393-d680-7cd6-b257-ce7248325463" xlink:to="loc_srt_RangeAxis_019f1393-d680-7f83-adac-fd79af324b09" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019f1393-d680-7f83-adac-fd79af324b09_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019f1393-d680-7f83-adac-fd79af324b09" xlink:to="loc_srt_RangeMember_019f1393-d680-7f83-adac-fd79af324b09_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019f1393-d680-7f60-a35f-3a4660b01e8f" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019f1393-d680-7f83-adac-fd79af324b09" xlink:to="loc_srt_RangeMember_019f1393-d680-7f60-a35f-3a4660b01e8f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019f1393-d680-779e-985e-d8382077c003" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019f1393-d680-7f60-a35f-3a4660b01e8f" xlink:to="loc_srt_MinimumMember_019f1393-d680-779e-985e-d8382077c003" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019f1393-d680-7142-b297-1b6765847a8d" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019f1393-d680-7f60-a35f-3a4660b01e8f" xlink:to="loc_srt_MaximumMember_019f1393-d680-7142-b297-1b6765847a8d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_StandardProductWarrantyNumberofYears_019f1393-d680-7822-8846-398a629fa43c" xlink:href="efoi-20260630.xsd#efoi_StandardProductWarrantyNumberofYears"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductWarrantyLiabilityLineItems_019f1393-d680-784a-9ab2-5bfe28ef2101" xlink:to="loc_efoi_StandardProductWarrantyNumberofYears_019f1393-d680-7822-8846-398a629fa43c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_ProductWarrantyNumberOfYearsForProductSold_019f1393-d680-7712-a216-db2f0b42bc28" xlink:href="efoi-20260630.xsd#efoi_ProductWarrantyNumberOfYearsForProductSold"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductWarrantyLiabilityLineItems_019f1393-d680-784a-9ab2-5bfe28ef2101" xlink:to="loc_efoi_ProductWarrantyNumberOfYearsForProductSold_019f1393-d680-7712-a216-db2f0b42bc28" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.energyfocusinc.com/role/BASISOFPRESENTATIONANDSUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESScheduleofNetSaleandAccountsReceivableDetails" xlink:type="simple" xlink:href="efoi-20260630.xsd#BASISOFPRESENTATIONANDSUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESScheduleofNetSaleandAccountsReceivableDetails"/>
  <link:definitionLink xlink:role="http://www.energyfocusinc.com/role/BASISOFPRESENTATIONANDSUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESScheduleofNetSaleandAccountsReceivableDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskLineItems_019f16f5-56af-7b5d-a373-7a120f4259a2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTable_019f16f5-56af-74bd-82d5-ee9ef3d463f0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_019f16f5-56af-7b5d-a373-7a120f4259a2" xlink:to="loc_us-gaap_ConcentrationRiskTable_019f16f5-56af-74bd-82d5-ee9ef3d463f0" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MajorCustomersAxis_019f16f5-56af-7224-9653-f4a28c9568e5" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MajorCustomersAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_019f16f5-56af-74bd-82d5-ee9ef3d463f0" xlink:to="loc_srt_MajorCustomersAxis_019f16f5-56af-7224-9653-f4a28c9568e5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_NameOfMajorCustomerDomain_019f16f5-56af-7224-9653-f4a28c9568e5_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_NameOfMajorCustomerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_MajorCustomersAxis_019f16f5-56af-7224-9653-f4a28c9568e5" xlink:to="loc_srt_NameOfMajorCustomerDomain_019f16f5-56af-7224-9653-f4a28c9568e5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_NameOfMajorCustomerDomain_019f16f5-56af-7885-ac3d-1e33c4f35a96" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_NameOfMajorCustomerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_MajorCustomersAxis_019f16f5-56af-7224-9653-f4a28c9568e5" xlink:to="loc_srt_NameOfMajorCustomerDomain_019f16f5-56af-7885-ac3d-1e33c4f35a96" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_CustomerAMember_019f16f5-56af-72b3-b3c5-d864ff40ef36" xlink:href="efoi-20260630.xsd#efoi_CustomerAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_019f16f5-56af-7885-ac3d-1e33c4f35a96" xlink:to="loc_efoi_CustomerAMember_019f16f5-56af-72b3-b3c5-d864ff40ef36" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_CustomerBMember_019f16f5-56af-7eb1-a616-f6d454be85bb" xlink:href="efoi-20260630.xsd#efoi_CustomerBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_019f16f5-56af-7885-ac3d-1e33c4f35a96" xlink:to="loc_efoi_CustomerBMember_019f16f5-56af-7eb1-a616-f6d454be85bb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_CustomerCMember_019f16f5-56af-79e8-9d22-3e8ac6c6b78e" xlink:href="efoi-20260630.xsd#efoi_CustomerCMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_019f16f5-56af-7885-ac3d-1e33c4f35a96" xlink:to="loc_efoi_CustomerCMember_019f16f5-56af-79e8-9d22-3e8ac6c6b78e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_CustomerDMember_019f16f5-56af-78e2-a41d-cce279f20845" xlink:href="efoi-20260630.xsd#efoi_CustomerDMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_019f16f5-56af-7885-ac3d-1e33c4f35a96" xlink:to="loc_efoi_CustomerDMember_019f16f5-56af-78e2-a41d-cce279f20845" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_CustomerEMember_019f16f5-56af-7c5e-886b-554f2cc8f90f" xlink:href="efoi-20260630.xsd#efoi_CustomerEMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_019f16f5-56af-7885-ac3d-1e33c4f35a96" xlink:to="loc_efoi_CustomerEMember_019f16f5-56af-7c5e-886b-554f2cc8f90f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_CustomerFMember_019f16f5-56af-7c92-a42e-d636c14e4b2a" xlink:href="efoi-20260630.xsd#efoi_CustomerFMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_019f16f5-56af-7885-ac3d-1e33c4f35a96" xlink:to="loc_efoi_CustomerFMember_019f16f5-56af-7c92-a42e-d636c14e4b2a" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_CustomerGMember_019f16f5-56af-7eec-bb00-a5301dc1f7f0" xlink:href="efoi-20260630.xsd#efoi_CustomerGMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_019f16f5-56af-7885-ac3d-1e33c4f35a96" xlink:to="loc_efoi_CustomerGMember_019f16f5-56af-7eec-bb00-a5301dc1f7f0" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019f16f5-56af-749c-89bb-b9971ddf63f2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskByBenchmarkAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_019f16f5-56af-74bd-82d5-ee9ef3d463f0" xlink:to="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019f16f5-56af-749c-89bb-b9971ddf63f2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019f16f5-56af-749c-89bb-b9971ddf63f2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019f16f5-56af-749c-89bb-b9971ddf63f2" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019f16f5-56af-749c-89bb-b9971ddf63f2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019f16f5-56af-7fe5-a1d7-d946a4503b7a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019f16f5-56af-749c-89bb-b9971ddf63f2" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019f16f5-56af-7fe5-a1d7-d946a4503b7a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SalesRevenueNetMember_019f16f5-56af-7f8f-b62d-831c373946f5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SalesRevenueNetMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019f16f5-56af-7fe5-a1d7-d946a4503b7a" xlink:to="loc_us-gaap_SalesRevenueNetMember_019f16f5-56af-7f8f-b62d-831c373946f5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableMember_019f16f5-f204-79cb-8522-d82091f7b30f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccountsReceivableMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019f16f5-56af-7fe5-a1d7-d946a4503b7a" xlink:to="loc_us-gaap_AccountsReceivableMember_019f16f5-f204-79cb-8522-d82091f7b30f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByTypeAxis_019f16f5-56af-7966-a1c5-edb66195e444" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_019f16f5-56af-74bd-82d5-ee9ef3d463f0" xlink:to="loc_us-gaap_ConcentrationRiskByTypeAxis_019f16f5-56af-7966-a1c5-edb66195e444" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_019f16f5-56af-7966-a1c5-edb66195e444_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_019f16f5-56af-7966-a1c5-edb66195e444" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_019f16f5-56af-7966-a1c5-edb66195e444_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_019f16f5-56af-7047-913f-96d7efd47b0a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_019f16f5-56af-7966-a1c5-edb66195e444" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_019f16f5-56af-7047-913f-96d7efd47b0a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerConcentrationRiskMember_019f16f5-56af-7d98-ad64-d4e3659e0f41" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CustomerConcentrationRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskTypeDomain_019f16f5-56af-7047-913f-96d7efd47b0a" xlink:to="loc_us-gaap_CustomerConcentrationRiskMember_019f16f5-56af-7d98-ad64-d4e3659e0f41" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskPercentage1_019f16f5-56af-7315-a66d-41a7514dd470" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskPercentage1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_019f16f5-56af-7b5d-a373-7a120f4259a2" xlink:to="loc_us-gaap_ConcentrationRiskPercentage1_019f16f5-56af-7315-a66d-41a7514dd470" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.energyfocusinc.com/role/BASISOFPRESENTATIONANDSUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESSchedulesofConcentrationofRiskTotalExpendituresandTradeAccountsPayableDetails" xlink:type="simple" xlink:href="efoi-20260630.xsd#BASISOFPRESENTATIONANDSUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESSchedulesofConcentrationofRiskTotalExpendituresandTradeAccountsPayableDetails"/>
  <link:definitionLink xlink:role="http://www.energyfocusinc.com/role/BASISOFPRESENTATIONANDSUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESSchedulesofConcentrationofRiskTotalExpendituresandTradeAccountsPayableDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskLineItems_019f1393-d680-71bb-a04a-895ccde62c70" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTable_019f1393-d680-77d6-80d0-a43eb01642cb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_019f1393-d680-71bb-a04a-895ccde62c70" xlink:to="loc_us-gaap_ConcentrationRiskTable_019f1393-d680-77d6-80d0-a43eb01642cb" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019f1393-d680-7d51-8b60-f30d3385a2d2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskByBenchmarkAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_019f1393-d680-77d6-80d0-a43eb01642cb" xlink:to="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019f1393-d680-7d51-8b60-f30d3385a2d2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019f1393-d680-7d51-8b60-f30d3385a2d2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019f1393-d680-7d51-8b60-f30d3385a2d2" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019f1393-d680-7d51-8b60-f30d3385a2d2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019f1393-d680-79fe-8c0a-104a37e2bb37" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019f1393-d680-7d51-8b60-f30d3385a2d2" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019f1393-d680-79fe-8c0a-104a37e2bb37" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_TotalExpendituresMember_019f1393-d680-7da2-a91d-940cbe74376a" xlink:href="efoi-20260630.xsd#efoi_TotalExpendituresMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019f1393-d680-79fe-8c0a-104a37e2bb37" xlink:to="loc_efoi_TotalExpendituresMember_019f1393-d680-7da2-a91d-940cbe74376a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsPayableMember_019f1393-d680-7e05-b9a3-868edc2a4cfe" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccountsPayableMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019f1393-d680-79fe-8c0a-104a37e2bb37" xlink:to="loc_us-gaap_AccountsPayableMember_019f1393-d680-7e05-b9a3-868edc2a4cfe" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByTypeAxis_019f1393-d680-7d3b-b662-32fe53391519" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_019f1393-d680-77d6-80d0-a43eb01642cb" xlink:to="loc_us-gaap_ConcentrationRiskByTypeAxis_019f1393-d680-7d3b-b662-32fe53391519" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_019f1393-d680-7d3b-b662-32fe53391519_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_019f1393-d680-7d3b-b662-32fe53391519" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_019f1393-d680-7d3b-b662-32fe53391519_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_019f1393-d680-79a1-9aa1-1d6235c8fe61" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_019f1393-d680-7d3b-b662-32fe53391519" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_019f1393-d680-79a1-9aa1-1d6235c8fe61" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SupplierConcentrationRiskMember_019f1393-d680-7773-a9f1-33d0261fc141" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SupplierConcentrationRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskTypeDomain_019f1393-d680-79a1-9aa1-1d6235c8fe61" xlink:to="loc_us-gaap_SupplierConcentrationRiskMember_019f1393-d680-7773-a9f1-33d0261fc141" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis_019f1393-d680-72cf-86f2-97b15c17ab3d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_019f1393-d680-77d6-80d0-a43eb01642cb" xlink:to="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis_019f1393-d680-72cf-86f2-97b15c17ab3d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain_019f1393-d680-72cf-86f2-97b15c17ab3d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis_019f1393-d680-72cf-86f2-97b15c17ab3d" xlink:to="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain_019f1393-d680-72cf-86f2-97b15c17ab3d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain_019f1393-d680-7eb6-a748-71c3e3c55089" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis_019f1393-d680-72cf-86f2-97b15c17ab3d" xlink:to="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain_019f1393-d680-7eb6-a748-71c3e3c55089" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_OffshoreSupplierAMember_019f1393-d680-7e5a-9638-ad6a1da93b9c" xlink:href="efoi-20260630.xsd#efoi_OffshoreSupplierAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain_019f1393-d680-7eb6-a748-71c3e3c55089" xlink:to="loc_efoi_OffshoreSupplierAMember_019f1393-d680-7e5a-9638-ad6a1da93b9c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_OffshoreSupplierBMember_019f7ea6-d453-7336-b6d5-e9f25866b2f8" xlink:href="efoi-20260630.xsd#efoi_OffshoreSupplierBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain_019f1393-d680-7eb6-a748-71c3e3c55089" xlink:to="loc_efoi_OffshoreSupplierBMember_019f7ea6-d453-7336-b6d5-e9f25866b2f8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_OffshoreSupplierCMember_019f7ea7-42ee-7e7a-b5e5-d1fd41d2314e" xlink:href="efoi-20260630.xsd#efoi_OffshoreSupplierCMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain_019f1393-d680-7eb6-a748-71c3e3c55089" xlink:to="loc_efoi_OffshoreSupplierCMember_019f7ea7-42ee-7e7a-b5e5-d1fd41d2314e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_OffshoreSupplierDAndEMember_019f7ea7-d22b-76bc-adb8-b6f5b7580a8a" xlink:href="efoi-20260630.xsd#efoi_OffshoreSupplierDAndEMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain_019f1393-d680-7eb6-a748-71c3e3c55089" xlink:to="loc_efoi_OffshoreSupplierDAndEMember_019f7ea7-d22b-76bc-adb8-b6f5b7580a8a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_OffshoreSupplierEMember_019fb9cc-8ec2-72ca-a056-048dde4ec132" xlink:href="efoi-20260630.xsd#efoi_OffshoreSupplierEMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain_019f1393-d680-7eb6-a748-71c3e3c55089" xlink:to="loc_efoi_OffshoreSupplierEMember_019fb9cc-8ec2-72ca-a056-048dde4ec132" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_OffshoreSupplierFMember_019f7eab-7423-7d40-a2b7-237f149d7939" xlink:href="efoi-20260630.xsd#efoi_OffshoreSupplierFMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain_019f1393-d680-7eb6-a748-71c3e3c55089" xlink:to="loc_efoi_OffshoreSupplierFMember_019f7eab-7423-7d40-a2b7-237f149d7939" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskPercentage1_019f1393-d680-7c2f-8e72-0048b84ddd28" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskPercentage1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_019f1393-d680-71bb-a04a-895ccde62c70" xlink:to="loc_us-gaap_ConcentrationRiskPercentage1_019f1393-d680-7c2f-8e72-0048b84ddd28" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.energyfocusinc.com/role/PROPERTYANDEQUIPMENTScheduleofPropertyandEquipmentDetails" xlink:type="simple" xlink:href="efoi-20260630.xsd#PROPERTYANDEQUIPMENTScheduleofPropertyandEquipmentDetails"/>
  <link:definitionLink xlink:role="http://www.energyfocusinc.com/role/PROPERTYANDEQUIPMENTScheduleofPropertyandEquipmentDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019f1393-d680-7fd0-bde7-bb5a74e5fda4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_019f1393-d680-7ab0-88b3-6df00d9a19f7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019f1393-d680-7fd0-bde7-bb5a74e5fda4" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_019f1393-d680-7ab0-88b3-6df00d9a19f7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019f1393-d680-730f-a847-b5e8eb119aa1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_019f1393-d680-7ab0-88b3-6df00d9a19f7" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019f1393-d680-730f-a847-b5e8eb119aa1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019f1393-d680-730f-a847-b5e8eb119aa1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019f1393-d680-730f-a847-b5e8eb119aa1" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019f1393-d680-730f-a847-b5e8eb119aa1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019f1393-d680-788a-ac6d-9c3fb2ab8ca6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019f1393-d680-730f-a847-b5e8eb119aa1" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019f1393-d680-788a-ac6d-9c3fb2ab8ca6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquipmentMember_019f1393-d680-7eaf-899d-cec33c312d0e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquipmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019f1393-d680-788a-ac6d-9c3fb2ab8ca6" xlink:to="loc_us-gaap_EquipmentMember_019f1393-d680-7eaf-899d-cec33c312d0e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_ToolingMember_019f1393-d680-72a8-9cd1-5b907135e65b" xlink:href="efoi-20260630.xsd#efoi_ToolingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019f1393-d680-788a-ac6d-9c3fb2ab8ca6" xlink:to="loc_efoi_ToolingMember_019f1393-d680-72a8-9cd1-5b907135e65b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VehiclesMember_019f1393-d680-7671-8fca-cd5605b7f20b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VehiclesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019f1393-d680-788a-ac6d-9c3fb2ab8ca6" xlink:to="loc_us-gaap_VehiclesMember_019f1393-d680-7671-8fca-cd5605b7f20b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseholdImprovementsMember_019f1393-d680-7cbb-ac54-d9c4092b8e72" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LeaseholdImprovementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019f1393-d680-788a-ac6d-9c3fb2ab8ca6" xlink:to="loc_us-gaap_LeaseholdImprovementsMember_019f1393-d680-7cbb-ac54-d9c4092b8e72" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019f1393-d680-707a-bffb-bc580dc2ba3d" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_019f1393-d680-7ab0-88b3-6df00d9a19f7" xlink:to="loc_srt_RangeAxis_019f1393-d680-707a-bffb-bc580dc2ba3d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019f1393-d680-707a-bffb-bc580dc2ba3d_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019f1393-d680-707a-bffb-bc580dc2ba3d" xlink:to="loc_srt_RangeMember_019f1393-d680-707a-bffb-bc580dc2ba3d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019f1393-d680-7b1f-b6cf-4fc327007f13" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019f1393-d680-707a-bffb-bc580dc2ba3d" xlink:to="loc_srt_RangeMember_019f1393-d680-7b1f-b6cf-4fc327007f13" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019f1393-d680-7d11-8621-b17fbfd71b2d" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019f1393-d680-7b1f-b6cf-4fc327007f13" xlink:to="loc_srt_MinimumMember_019f1393-d680-7d11-8621-b17fbfd71b2d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019f1393-d680-7161-a3d0-71cc7333cd90" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019f1393-d680-7b1f-b6cf-4fc327007f13" xlink:to="loc_srt_MaximumMember_019f1393-d680-7161-a3d0-71cc7333cd90" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentUsefulLife_019f1393-d680-7bce-954c-751469829d20" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019f1393-d680-7fd0-bde7-bb5a74e5fda4" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentUsefulLife_019f1393-d680-7bce-954c-751469829d20" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentGross_019f1393-d680-7596-9037-fc866841918c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019f1393-d680-7fd0-bde7-bb5a74e5fda4" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentGross_019f1393-d680-7596-9037-fc866841918c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetBeforeAccumulatedDepreciationAndAmortization_019f1393-d680-7aac-948c-96db85b1e01f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetBeforeAccumulatedDepreciationAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019f1393-d680-7fd0-bde7-bb5a74e5fda4" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetBeforeAccumulatedDepreciationAndAmortization_019f1393-d680-7aac-948c-96db85b1e01f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAccumulatedDepreciationAndAmortization_019f1393-d680-75b4-9da6-f03cd89ecabf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAccumulatedDepreciationAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019f1393-d680-7fd0-bde7-bb5a74e5fda4" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAccumulatedDepreciationAndAmortization_019f1393-d680-75b4-9da6-f03cd89ecabf" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization_019f1393-d680-73f2-9110-3bf4ab9cd5e4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019f1393-d680-7fd0-bde7-bb5a74e5fda4" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization_019f1393-d680-73f2-9110-3bf4ab9cd5e4" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.energyfocusinc.com/role/DEBTNarrativeDetails" xlink:type="simple" xlink:href="efoi-20260630.xsd#DEBTNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.energyfocusinc.com/role/DEBTNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermDebtLineItems_019f5b76-f9c7-7083-9859-981ef5ce723d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShortTermDebtLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShortTermDebtTable_019f5b76-f9c7-7b87-bd71-f964125142ff" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShortTermDebtTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShortTermDebtLineItems_019f5b76-f9c7-7083-9859-981ef5ce723d" xlink:to="loc_us-gaap_ScheduleOfShortTermDebtTable_019f5b76-f9c7-7b87-bd71-f964125142ff" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermDebtTypeAxis_019f5b76-f9c7-7175-a38a-edd247859e4e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShortTermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShortTermDebtTable_019f5b76-f9c7-7b87-bd71-f964125142ff" xlink:to="loc_us-gaap_ShortTermDebtTypeAxis_019f5b76-f9c7-7175-a38a-edd247859e4e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermDebtTypeDomain_019f5b76-f9c7-7175-a38a-edd247859e4e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShortTermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ShortTermDebtTypeAxis_019f5b76-f9c7-7175-a38a-edd247859e4e" xlink:to="loc_us-gaap_ShortTermDebtTypeDomain_019f5b76-f9c7-7175-a38a-edd247859e4e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermDebtTypeDomain_019f5b76-f9c7-789f-bb1e-29118bcd346a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShortTermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ShortTermDebtTypeAxis_019f5b76-f9c7-7175-a38a-edd247859e4e" xlink:to="loc_us-gaap_ShortTermDebtTypeDomain_019f5b76-f9c7-789f-bb1e-29118bcd346a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermDebtMember_019f5b76-f9c7-7a77-a1be-808ccd9631c2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShortTermDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtTypeDomain_019f5b76-f9c7-789f-bb1e-29118bcd346a" xlink:to="loc_us-gaap_ShortTermDebtMember_019f5b76-f9c7-7a77-a1be-808ccd9631c2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_019f5b76-f9c7-737d-ad31-15e18ce6e976" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtLineItems_019f5b76-f9c7-7083-9859-981ef5ce723d" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_019f5b76-f9c7-737d-ad31-15e18ce6e976" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermBorrowings_019f5b76-f9c7-7ca9-8736-8c336a416465" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShortTermBorrowings"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtLineItems_019f5b76-f9c7-7083-9859-981ef5ce723d" xlink:to="loc_us-gaap_ShortTermBorrowings_019f5b76-f9c7-7ca9-8736-8c336a416465" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019fa7a7-8bae-7256-aae0-a65197740ec7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtLineItems_019f5b76-f9c7-7083-9859-981ef5ce723d" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019fa7a7-8bae-7256-aae0-a65197740ec7" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.energyfocusinc.com/role/INCOMETAXESDetails" xlink:type="simple" xlink:href="efoi-20260630.xsd#INCOMETAXESDetails"/>
  <link:definitionLink xlink:role="http://www.energyfocusinc.com/role/INCOMETAXESDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLossCarryforwardsLineItems_019f1393-d680-729e-a9f0-46aa689f73b7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingLossCarryforwardsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLossCarryforwardsTable_019f1393-d680-79d6-a6f5-d44704e896df" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingLossCarryforwardsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_019f1393-d680-729e-a9f0-46aa689f73b7" xlink:to="loc_us-gaap_OperatingLossCarryforwardsTable_019f1393-d680-79d6-a6f5-d44704e896df" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityAxis_019f1393-d680-7690-ad30-f88598fa2e12" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeTaxAuthorityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_OperatingLossCarryforwardsTable_019f1393-d680-79d6-a6f5-d44704e896df" xlink:to="loc_us-gaap_IncomeTaxAuthorityAxis_019f1393-d680-7690-ad30-f88598fa2e12" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_019f1393-d680-7690-ad30-f88598fa2e12_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeTaxAuthorityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_019f1393-d680-7690-ad30-f88598fa2e12" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_019f1393-d680-7690-ad30-f88598fa2e12_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_019f1393-d680-7613-b632-96e29571dd5b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeTaxAuthorityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_019f1393-d680-7690-ad30-f88598fa2e12" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_019f1393-d680-7613-b632-96e29571dd5b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DomesticCountryMember_019f1393-d680-7fd4-abb0-d432f3033e72" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DomesticCountryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_019f1393-d680-7613-b632-96e29571dd5b" xlink:to="loc_us-gaap_DomesticCountryMember_019f1393-d680-7fd4-abb0-d432f3033e72" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StateAndLocalJurisdictionMember_019f1393-d680-7efb-be54-40e1b22bff25" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StateAndLocalJurisdictionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_019f1393-d680-7613-b632-96e29571dd5b" xlink:to="loc_us-gaap_StateAndLocalJurisdictionMember_019f1393-d680-7efb-be54-40e1b22bff25" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_019fd1b3-0ed9-7106-8659-0e249d043e13" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeTaxExpenseBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_019f1393-d680-729e-a9f0-46aa689f73b7" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_019fd1b3-0ed9-7106-8659-0e249d043e13" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredTaxAssetsOperatingLossCarryforwards_019f1393-d680-70de-a352-b455ddf130c4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DeferredTaxAssetsOperatingLossCarryforwards"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_019f1393-d680-729e-a9f0-46aa689f73b7" xlink:to="loc_us-gaap_DeferredTaxAssetsOperatingLossCarryforwards_019f1393-d680-70de-a352-b455ddf130c4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_DeferredTaxAssetsOperatingLossCarryforwardsPortionAvailableAfterApplicationOfIRCSection382Limitations_019f1393-d680-782a-b83f-ccaf753bc272" xlink:href="efoi-20260630.xsd#efoi_DeferredTaxAssetsOperatingLossCarryforwardsPortionAvailableAfterApplicationOfIRCSection382Limitations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_019f1393-d680-729e-a9f0-46aa689f73b7" xlink:to="loc_efoi_DeferredTaxAssetsOperatingLossCarryforwardsPortionAvailableAfterApplicationOfIRCSection382Limitations_019f1393-d680-782a-b83f-ccaf753bc272" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLossCarryforwards_019f1393-d680-74f1-b17d-eba5d54e24a7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingLossCarryforwards"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_019f1393-d680-729e-a9f0-46aa689f73b7" xlink:to="loc_us-gaap_OperatingLossCarryforwards_019f1393-d680-74f1-b17d-eba5d54e24a7" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsSubjectToExpiration_019f1393-d680-726b-a1ca-8b8eff1e3934" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsSubjectToExpiration"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_019f1393-d680-729e-a9f0-46aa689f73b7" xlink:to="loc_us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsSubjectToExpiration_019f1393-d680-726b-a1ca-8b8eff1e3934" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.energyfocusinc.com/role/STOCKHOLDERSEQUITYPrivatePlacementsDetails" xlink:type="simple" xlink:href="efoi-20260630.xsd#STOCKHOLDERSEQUITYPrivatePlacementsDetails"/>
  <link:definitionLink xlink:role="http://www.energyfocusinc.com/role/STOCKHOLDERSEQUITYPrivatePlacementsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f1393-d680-7dc2-a17b-1d4bed46aee8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019f1393-d680-7b80-94e6-bf0810a2c595" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f1393-d680-7dc2-a17b-1d4bed46aee8" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019f1393-d680-7b80-94e6-bf0810a2c595" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_019f1393-d680-757c-b744-66cfcf1d7a37" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsidiarySaleOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019f1393-d680-7b80-94e6-bf0810a2c595" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_019f1393-d680-757c-b744-66cfcf1d7a37" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f1393-d680-757c-b744-66cfcf1d7a37_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019f1393-d680-757c-b744-66cfcf1d7a37" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f1393-d680-757c-b744-66cfcf1d7a37_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f1393-d680-7df5-a65d-309ca22730d2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019f1393-d680-757c-b744-66cfcf1d7a37" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f1393-d680-7df5-a65d-309ca22730d2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrivatePlacementMember_019f1393-d680-7a62-bd0c-9efb968dc0b2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PrivatePlacementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f1393-d680-7df5-a65d-309ca22730d2" xlink:to="loc_us-gaap_PrivatePlacementMember_019f1393-d680-7a62-bd0c-9efb968dc0b2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_019f1393-d680-7af6-b89f-258174d5aa20" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f1393-d680-7dc2-a17b-1d4bed46aee8" xlink:to="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_019f1393-d680-7af6-b89f-258174d5aa20" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.energyfocusinc.com/role/STOCKHOLDERSEQUITYMay2026PrivatePlacementDetails" xlink:type="simple" xlink:href="efoi-20260630.xsd#STOCKHOLDERSEQUITYMay2026PrivatePlacementDetails"/>
  <link:definitionLink xlink:role="http://www.energyfocusinc.com/role/STOCKHOLDERSEQUITYMay2026PrivatePlacementDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f173c-aebc-74f1-aa42-53ae34b54236" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019f173c-aebc-78e8-8790-42154022a48a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f173c-aebc-74f1-aa42-53ae34b54236" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019f173c-aebc-78e8-8790-42154022a48a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_019f173c-aebc-7e39-bca7-dbbc73ac1102" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsidiarySaleOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019f173c-aebc-78e8-8790-42154022a48a" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_019f173c-aebc-7e39-bca7-dbbc73ac1102" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f173c-aebc-7e39-bca7-dbbc73ac1102_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019f173c-aebc-7e39-bca7-dbbc73ac1102" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f173c-aebc-7e39-bca7-dbbc73ac1102_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f2583-22db-7fc9-a009-64eb240de27c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019f173c-aebc-7e39-bca7-dbbc73ac1102" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f2583-22db-7fc9-a009-64eb240de27c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_May2026PrivatePlacementMember_019f2583-22dc-7d88-ba6e-8c80db97d7f1" xlink:href="efoi-20260630.xsd#efoi_May2026PrivatePlacementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f2583-22db-7fc9-a009-64eb240de27c" xlink:to="loc_efoi_May2026PrivatePlacementMember_019f2583-22dc-7d88-ba6e-8c80db97d7f1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_019f173c-aebc-7156-bb2b-039aa77159f8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f173c-aebc-74f1-aa42-53ae34b54236" xlink:to="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_019f173c-aebc-7156-bb2b-039aa77159f8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_019f173c-aebc-7ccb-adf2-f4a008b8d41e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f173c-aebc-74f1-aa42-53ae34b54236" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_019f173c-aebc-7ccb-adf2-f4a008b8d41e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockPricePerShare_019f173c-aebc-734f-a809-2808d30f5297" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockPricePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f173c-aebc-74f1-aa42-53ae34b54236" xlink:to="loc_us-gaap_SaleOfStockPricePerShare_019f173c-aebc-734f-a809-2808d30f5297" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfPrivatePlacement_019f173c-aebc-7704-81b9-b939f774acb8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromIssuanceOfPrivatePlacement"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f173c-aebc-74f1-aa42-53ae34b54236" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfPrivatePlacement_019f173c-aebc-7704-81b9-b939f774acb8" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.energyfocusinc.com/role/STOCKHOLDERSEQUITYNovember2025PrivatePlacementDetails" xlink:type="simple" xlink:href="efoi-20260630.xsd#STOCKHOLDERSEQUITYNovember2025PrivatePlacementDetails"/>
  <link:definitionLink xlink:role="http://www.energyfocusinc.com/role/STOCKHOLDERSEQUITYNovember2025PrivatePlacementDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f173e-8640-7fdd-b62b-3e2a62c573d1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019f173e-8640-7132-ae61-2a06db12335d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f173e-8640-7fdd-b62b-3e2a62c573d1" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019f173e-8640-7132-ae61-2a06db12335d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_019f173e-8640-71c2-b5c4-6153b2ce8939" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsidiarySaleOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019f173e-8640-7132-ae61-2a06db12335d" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_019f173e-8640-71c2-b5c4-6153b2ce8939" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f173e-8640-71c2-b5c4-6153b2ce8939_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019f173e-8640-71c2-b5c4-6153b2ce8939" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f173e-8640-71c2-b5c4-6153b2ce8939_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f173e-8640-7471-a2ab-86f2c1ca7b6d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019f173e-8640-71c2-b5c4-6153b2ce8939" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f173e-8640-7471-a2ab-86f2c1ca7b6d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_November2025PrivatePlacementInitialSharesMember_019f173e-8640-7c2a-bab1-2c257509d50e" xlink:href="efoi-20260630.xsd#efoi_November2025PrivatePlacementInitialSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f173e-8640-7471-a2ab-86f2c1ca7b6d" xlink:to="loc_efoi_November2025PrivatePlacementInitialSharesMember_019f173e-8640-7c2a-bab1-2c257509d50e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_November2025PrivatePlacementMember_019f173e-8640-743b-a048-29928a22c5cd" xlink:href="efoi-20260630.xsd#efoi_November2025PrivatePlacementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f173e-8640-7471-a2ab-86f2c1ca7b6d" xlink:to="loc_efoi_November2025PrivatePlacementMember_019f173e-8640-743b-a048-29928a22c5cd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_November2025PrivatePlacementAggregateSharesMember_019f173e-8640-7adb-96a0-38595d986ead" xlink:href="efoi-20260630.xsd#efoi_November2025PrivatePlacementAggregateSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f173e-8640-7471-a2ab-86f2c1ca7b6d" xlink:to="loc_efoi_November2025PrivatePlacementAggregateSharesMember_019f173e-8640-7adb-96a0-38595d986ead" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_019f173e-8640-7567-a8b0-9c61c20b482f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f173e-8640-7fdd-b62b-3e2a62c573d1" xlink:to="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_019f173e-8640-7567-a8b0-9c61c20b482f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_019f173e-8640-714c-bc9a-25d654291292" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f173e-8640-7fdd-b62b-3e2a62c573d1" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_019f173e-8640-714c-bc9a-25d654291292" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockPricePerShare_019f173e-8640-734c-bdd6-be16c8e12804" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockPricePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f173e-8640-7fdd-b62b-3e2a62c573d1" xlink:to="loc_us-gaap_SaleOfStockPricePerShare_019f173e-8640-734c-bdd6-be16c8e12804" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfPrivatePlacement_019f173e-8640-7a58-afa5-097eafd4491c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromIssuanceOfPrivatePlacement"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f173e-8640-7fdd-b62b-3e2a62c573d1" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfPrivatePlacement_019f173e-8640-7a58-afa5-097eafd4491c" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.energyfocusinc.com/role/STOCKHOLDERSEQUITYAugust2025PrivatePlacementDetails" xlink:type="simple" xlink:href="efoi-20260630.xsd#STOCKHOLDERSEQUITYAugust2025PrivatePlacementDetails"/>
  <link:definitionLink xlink:role="http://www.energyfocusinc.com/role/STOCKHOLDERSEQUITYAugust2025PrivatePlacementDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f173f-1342-73ae-9174-be27ec2cc68a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019f173f-1342-7927-9dd5-6495f49888c3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f173f-1342-73ae-9174-be27ec2cc68a" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019f173f-1342-7927-9dd5-6495f49888c3" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_019f173f-1342-7e79-bfcf-804f8c7b95db" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsidiarySaleOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019f173f-1342-7927-9dd5-6495f49888c3" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_019f173f-1342-7e79-bfcf-804f8c7b95db" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f173f-1342-7e79-bfcf-804f8c7b95db_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019f173f-1342-7e79-bfcf-804f8c7b95db" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f173f-1342-7e79-bfcf-804f8c7b95db_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f173f-1342-73a7-a80e-7437960a5b50" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019f173f-1342-7e79-bfcf-804f8c7b95db" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f173f-1342-73a7-a80e-7437960a5b50" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_August2025PrivatePlacementMember_019f173f-1342-7226-bb86-92fc88820d2e" xlink:href="efoi-20260630.xsd#efoi_August2025PrivatePlacementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f173f-1342-73a7-a80e-7437960a5b50" xlink:to="loc_efoi_August2025PrivatePlacementMember_019f173f-1342-7226-bb86-92fc88820d2e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_019f173f-1342-78fe-8350-0252e1b5bb46" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f173f-1342-73ae-9174-be27ec2cc68a" xlink:to="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_019f173f-1342-78fe-8350-0252e1b5bb46" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_019f173f-1342-78c8-b6f4-15ed5ab20264" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f173f-1342-73ae-9174-be27ec2cc68a" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_019f173f-1342-78c8-b6f4-15ed5ab20264" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockPricePerShare_019f173f-1342-7f7d-9a24-606a90725a01" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockPricePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f173f-1342-73ae-9174-be27ec2cc68a" xlink:to="loc_us-gaap_SaleOfStockPricePerShare_019f173f-1342-7f7d-9a24-606a90725a01" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfPrivatePlacement_019f173f-1342-704a-8428-e8bf9445eebf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromIssuanceOfPrivatePlacement"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f173f-1342-73ae-9174-be27ec2cc68a" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfPrivatePlacement_019f173f-1342-704a-8428-e8bf9445eebf" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.energyfocusinc.com/role/STOCKHOLDERSEQUITYJune2025PrivatePlacementDetails" xlink:type="simple" xlink:href="efoi-20260630.xsd#STOCKHOLDERSEQUITYJune2025PrivatePlacementDetails"/>
  <link:definitionLink xlink:role="http://www.energyfocusinc.com/role/STOCKHOLDERSEQUITYJune2025PrivatePlacementDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f1393-d680-79d1-867d-6d9e67d33fc2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019f1393-d680-759c-ada7-416846443600" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f1393-d680-79d1-867d-6d9e67d33fc2" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019f1393-d680-759c-ada7-416846443600" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_019f1393-d680-7aec-b26c-597dd08b503b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsidiarySaleOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019f1393-d680-759c-ada7-416846443600" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_019f1393-d680-7aec-b26c-597dd08b503b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f1393-d680-7aec-b26c-597dd08b503b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019f1393-d680-7aec-b26c-597dd08b503b" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f1393-d680-7aec-b26c-597dd08b503b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f1393-d680-71e5-947d-bc6f051a9ed8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019f1393-d680-7aec-b26c-597dd08b503b" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f1393-d680-71e5-947d-bc6f051a9ed8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_June2025PrivatePlacementMember_019f1393-d680-749f-bc10-bc633e912b9c" xlink:href="efoi-20260630.xsd#efoi_June2025PrivatePlacementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f1393-d680-71e5-947d-bc6f051a9ed8" xlink:to="loc_efoi_June2025PrivatePlacementMember_019f1393-d680-749f-bc10-bc633e912b9c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_019f1393-d680-7cd2-90eb-0f72dba1ce40" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f1393-d680-79d1-867d-6d9e67d33fc2" xlink:to="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_019f1393-d680-7cd2-90eb-0f72dba1ce40" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_019f1393-d680-7513-8e96-51f49aab1357" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f1393-d680-79d1-867d-6d9e67d33fc2" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_019f1393-d680-7513-8e96-51f49aab1357" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockPricePerShare_019f1393-d680-773e-b2e1-1270f8636db8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockPricePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f1393-d680-79d1-867d-6d9e67d33fc2" xlink:to="loc_us-gaap_SaleOfStockPricePerShare_019f1393-d680-773e-b2e1-1270f8636db8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfPrivatePlacement_019f1393-d680-723a-b33a-88124e8894b7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromIssuanceOfPrivatePlacement"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f1393-d680-79d1-867d-6d9e67d33fc2" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfPrivatePlacement_019f1393-d680-723a-b33a-88124e8894b7" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.energyfocusinc.com/role/STOCKHOLDERSEQUITYMarch2025privateplacementDetails" xlink:type="simple" xlink:href="efoi-20260630.xsd#STOCKHOLDERSEQUITYMarch2025privateplacementDetails"/>
  <link:definitionLink xlink:role="http://www.energyfocusinc.com/role/STOCKHOLDERSEQUITYMarch2025privateplacementDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f1393-d680-7b8b-ab33-62a505339754" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019f1393-d680-75b6-a34e-615eae4040f0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f1393-d680-7b8b-ab33-62a505339754" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019f1393-d680-75b6-a34e-615eae4040f0" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_019f1393-d680-7588-8db0-093d41f427c3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsidiarySaleOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019f1393-d680-75b6-a34e-615eae4040f0" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_019f1393-d680-7588-8db0-093d41f427c3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f1393-d680-7588-8db0-093d41f427c3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019f1393-d680-7588-8db0-093d41f427c3" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f1393-d680-7588-8db0-093d41f427c3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f1393-d680-7325-b2f2-f191bd8bb39e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019f1393-d680-7588-8db0-093d41f427c3" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f1393-d680-7325-b2f2-f191bd8bb39e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_March2025PrivatePlacementMember_019f1393-d680-7ec0-bbfc-95b545d94994" xlink:href="efoi-20260630.xsd#efoi_March2025PrivatePlacementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f1393-d680-7325-b2f2-f191bd8bb39e" xlink:to="loc_efoi_March2025PrivatePlacementMember_019f1393-d680-7ec0-bbfc-95b545d94994" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_019f1393-d680-7981-a866-b869d0c690d4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f1393-d680-7b8b-ab33-62a505339754" xlink:to="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_019f1393-d680-7981-a866-b869d0c690d4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_019f1393-d680-7fc2-9976-42a20d62b223" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f1393-d680-7b8b-ab33-62a505339754" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_019f1393-d680-7fc2-9976-42a20d62b223" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockPricePerShare_019f1393-d680-75e9-8135-24274ed1c2b1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockPricePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f1393-d680-7b8b-ab33-62a505339754" xlink:to="loc_us-gaap_SaleOfStockPricePerShare_019f1393-d680-75e9-8135-24274ed1c2b1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfPrivatePlacement_019f1393-d680-734b-9554-b2be69133c9f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromIssuanceOfPrivatePlacement"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f1393-d680-7b8b-ab33-62a505339754" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfPrivatePlacement_019f1393-d680-734b-9554-b2be69133c9f" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.energyfocusinc.com/role/STOCKHOLDERSEQUITYPreferredStockDetails" xlink:type="simple" xlink:href="efoi-20260630.xsd#STOCKHOLDERSEQUITYPreferredStockDetails"/>
  <link:definitionLink xlink:role="http://www.energyfocusinc.com/role/STOCKHOLDERSEQUITYPreferredStockDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f1393-d680-7834-ba88-810cc4feee0f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019f1393-d680-7306-974d-d71ec6d44936" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f1393-d680-7834-ba88-810cc4feee0f" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019f1393-d680-7306-974d-d71ec6d44936" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019f1393-d680-73da-9bf4-6b179962484e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019f1393-d680-7306-974d-d71ec6d44936" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019f1393-d680-73da-9bf4-6b179962484e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019f1393-d680-73da-9bf4-6b179962484e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019f1393-d680-73da-9bf4-6b179962484e" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019f1393-d680-73da-9bf4-6b179962484e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019f1393-d680-79aa-98e5-e9a7cae53891" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019f1393-d680-73da-9bf4-6b179962484e" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019f1393-d680-79aa-98e5-e9a7cae53891" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtMember_019f1393-d680-70c1-879d-69e92f6184a0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConvertibleDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019f1393-d680-79aa-98e5-e9a7cae53891" xlink:to="loc_us-gaap_ConvertibleDebtMember_019f1393-d680-70c1-879d-69e92f6184a0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_019f1393-d680-7a8e-a625-d89abb04eed4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019f1393-d680-7306-974d-d71ec6d44936" xlink:to="loc_us-gaap_StatementClassOfStockAxis_019f1393-d680-7a8e-a625-d89abb04eed4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019f1393-d680-7a8e-a625-d89abb04eed4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019f1393-d680-7a8e-a625-d89abb04eed4" xlink:to="loc_us-gaap_ClassOfStockDomain_019f1393-d680-7a8e-a625-d89abb04eed4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019f1393-d680-72c8-a37a-df01f9bf729f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019f1393-d680-7a8e-a625-d89abb04eed4" xlink:to="loc_us-gaap_ClassOfStockDomain_019f1393-d680-72c8-a37a-df01f9bf729f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertiblePreferredStockMember_019f1393-d680-7fc8-86ad-2ff73f27ed22" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConvertiblePreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019f1393-d680-72c8-a37a-df01f9bf729f" xlink:to="loc_us-gaap_ConvertiblePreferredStockMember_019f1393-d680-7fc8-86ad-2ff73f27ed22" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019f1393-d680-7f4e-b637-971d4dc20b8e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019f1393-d680-7306-974d-d71ec6d44936" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019f1393-d680-7f4e-b637-971d4dc20b8e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019f1393-d680-7f4e-b637-971d4dc20b8e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019f1393-d680-7f4e-b637-971d4dc20b8e" xlink:to="loc_us-gaap_EquityComponentDomain_019f1393-d680-7f4e-b637-971d4dc20b8e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019f1393-d680-7e78-84e4-f44bf51d3c57" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019f1393-d680-7f4e-b637-971d4dc20b8e" xlink:to="loc_us-gaap_EquityComponentDomain_019f1393-d680-7e78-84e4-f44bf51d3c57" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_019f1393-d680-74c2-9efb-19faddbf9656" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019f1393-d680-7e78-84e4-f44bf51d3c57" xlink:to="loc_us-gaap_CommonStockMember_019f1393-d680-74c2-9efb-19faddbf9656" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesAuthorized_019f1393-d680-7b50-a57e-3e1a1d10b77c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PreferredStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f1393-d680-7834-ba88-810cc4feee0f" xlink:to="loc_us-gaap_PreferredStockSharesAuthorized_019f1393-d680-7b50-a57e-3e1a1d10b77c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_019f1393-d680-7228-b0b9-d8b1c1de9818" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f1393-d680-7834-ba88-810cc4feee0f" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_019f1393-d680-7228-b0b9-d8b1c1de9818" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_PercentageOfSeriesAPreferredStockEligibleToVote_019f1393-d680-7681-8f58-62aeb4ecf770" xlink:href="efoi-20260630.xsd#efoi_PercentageOfSeriesAPreferredStockEligibleToVote"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f1393-d680-7834-ba88-810cc4feee0f" xlink:to="loc_efoi_PercentageOfSeriesAPreferredStockEligibleToVote_019f1393-d680-7681-8f58-62aeb4ecf770" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentConvertibleConversionPrice1_019f1393-d680-7a56-bb06-4937312d9145" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentConvertibleConversionPrice1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f1393-d680-7834-ba88-810cc4feee0f" xlink:to="loc_us-gaap_DebtInstrumentConvertibleConversionPrice1_019f1393-d680-7a56-bb06-4937312d9145" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityNoteStockSplitConversionRatio1_019f1393-d680-7388-a4a8-fa6a770ad50f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquityNoteStockSplitConversionRatio1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f1393-d680-7834-ba88-810cc4feee0f" xlink:to="loc_us-gaap_StockholdersEquityNoteStockSplitConversionRatio1_019f1393-d680-7388-a4a8-fa6a770ad50f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesIssued_019f1393-d680-79aa-a10e-398b40de0629" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PreferredStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f1393-d680-7834-ba88-810cc4feee0f" xlink:to="loc_us-gaap_PreferredStockSharesIssued_019f1393-d680-79aa-a10e-398b40de0629" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesOutstanding_019f1393-d680-759d-b69d-f35df89b903f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PreferredStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f1393-d680-7834-ba88-810cc4feee0f" xlink:to="loc_us-gaap_PreferredStockSharesOutstanding_019f1393-d680-759d-b69d-f35df89b903f" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion_019f1393-d680-7af6-be63-352bc8521408" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f1393-d680-7834-ba88-810cc4feee0f" xlink:to="loc_us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion_019f1393-d680-7af6-be63-352bc8521408" xlink:type="arc" order="7"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.energyfocusinc.com/role/STOCKHOLDERSEQUITYScheduleofStockholdersEquityNoteWarrantsorRightsDetails" xlink:type="simple" xlink:href="efoi-20260630.xsd#STOCKHOLDERSEQUITYScheduleofStockholdersEquityNoteWarrantsorRightsDetails"/>
  <link:definitionLink xlink:role="http://www.energyfocusinc.com/role/STOCKHOLDERSEQUITYScheduleofStockholdersEquityNoteWarrantsorRightsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f1393-d680-7895-92da-1c3fe1ab6f2a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019f1393-d680-75af-8a00-bd5c31cb24f1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f1393-d680-7895-92da-1c3fe1ab6f2a" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019f1393-d680-75af-8a00-bd5c31cb24f1" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_019f1393-d680-708c-873f-f97860e75b6e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsidiarySaleOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019f1393-d680-75af-8a00-bd5c31cb24f1" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_019f1393-d680-708c-873f-f97860e75b6e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f1393-d680-708c-873f-f97860e75b6e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019f1393-d680-708c-873f-f97860e75b6e" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f1393-d680-708c-873f-f97860e75b6e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f1393-d680-7090-95c1-f3f2f4ed7118" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019f1393-d680-708c-873f-f97860e75b6e" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f1393-d680-7090-95c1-f3f2f4ed7118" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_June2022WarrantsMember_019f1393-d680-70af-a123-78ce841140c1" xlink:href="efoi-20260630.xsd#efoi_June2022WarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f1393-d680-7090-95c1-f3f2f4ed7118" xlink:to="loc_efoi_June2022WarrantsMember_019f1393-d680-70af-a123-78ce841140c1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_December2021WarrantsMember_019f1393-d680-7ca0-bc34-97a7528bb6b4" xlink:href="efoi-20260630.xsd#efoi_December2021WarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f1393-d680-7090-95c1-f3f2f4ed7118" xlink:to="loc_efoi_December2021WarrantsMember_019f1393-d680-7ca0-bc34-97a7528bb6b4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_ClassOfWarrantOrRightWarrantsIssued_019f1393-d680-773b-9207-8104cf29a2ef" xlink:href="efoi-20260630.xsd#efoi_ClassOfWarrantOrRightWarrantsIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f1393-d680-7895-92da-1c3fe1ab6f2a" xlink:to="loc_efoi_ClassOfWarrantOrRightWarrantsIssued_019f1393-d680-773b-9207-8104cf29a2ef" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockPricePerShare_019f1393-d680-778f-b317-f489bfcf31dd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockPricePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f1393-d680-7895-92da-1c3fe1ab6f2a" xlink:to="loc_us-gaap_SaleOfStockPricePerShare_019f1393-d680-778f-b317-f489bfcf31dd" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.energyfocusinc.com/role/STOCKHOLDERSEQUITYScheduleofStockBasedCompensationExpenseDetails" xlink:type="simple" xlink:href="efoi-20260630.xsd#STOCKHOLDERSEQUITYScheduleofStockBasedCompensationExpenseDetails"/>
  <link:definitionLink xlink:role="http://www.energyfocusinc.com/role/STOCKHOLDERSEQUITYScheduleofStockBasedCompensationExpenseDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_019f1393-d680-7b17-a00d-20355044fc77" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_019f1393-d680-7f23-8d92-63445a9b9ef7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_019f1393-d680-7b17-a00d-20355044fc77" xlink:to="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_019f1393-d680-7f23-8d92-63445a9b9ef7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementOfIncomeLocationBalanceAxis_019f1393-d680-734f-b658-809da319873a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementOfIncomeLocationBalanceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_019f1393-d680-7f23-8d92-63445a9b9ef7" xlink:to="loc_us-gaap_StatementOfIncomeLocationBalanceAxis_019f1393-d680-734f-b658-809da319873a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_019f1393-d680-7865-b307-24307109cd9c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_019f1393-d680-7b17-a00d-20355044fc77" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_019f1393-d680-7865-b307-24307109cd9c" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.energyfocusinc.com/role/STOCKHOLDERSEQUITYScheduleofStockBasedCompensationExpenseDetails_1" xlink:type="simple" xlink:href="efoi-20260630.xsd#STOCKHOLDERSEQUITYScheduleofStockBasedCompensationExpenseDetails_1"/>
  <link:definitionLink xlink:role="http://www.energyfocusinc.com/role/STOCKHOLDERSEQUITYScheduleofStockBasedCompensationExpenseDetails_1" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_019f1393-d680-7b17-a00d-20355044fc77" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_019f1393-d680-7f23-8d92-63445a9b9ef7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_019f1393-d680-7b17-a00d-20355044fc77" xlink:to="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_019f1393-d680-7f23-8d92-63445a9b9ef7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_019f1393-d680-7865-b307-24307109cd9c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_019f1393-d680-7b17-a00d-20355044fc77" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_019f1393-d680-7865-b307-24307109cd9c" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.energyfocusinc.com/role/STOCKHOLDERSEQUITYStockOptionsNarrativeDetails" xlink:type="simple" xlink:href="efoi-20260630.xsd#STOCKHOLDERSEQUITYStockOptionsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.energyfocusinc.com/role/STOCKHOLDERSEQUITYStockOptionsNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f1393-d680-7d15-a0c9-ec0e0d55c7b7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019f1393-d680-72e4-8a51-cfd2021b06a1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f1393-d680-7d15-a0c9-ec0e0d55c7b7" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019f1393-d680-72e4-8a51-cfd2021b06a1" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019f1393-d680-743f-ab07-8b582f03b5aa" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019f1393-d680-72e4-8a51-cfd2021b06a1" xlink:to="loc_srt_RangeAxis_019f1393-d680-743f-ab07-8b582f03b5aa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019f1393-d680-743f-ab07-8b582f03b5aa_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019f1393-d680-743f-ab07-8b582f03b5aa" xlink:to="loc_srt_RangeMember_019f1393-d680-743f-ab07-8b582f03b5aa_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019f1393-d680-75df-96d6-3eddbd1735fc" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019f1393-d680-743f-ab07-8b582f03b5aa" xlink:to="loc_srt_RangeMember_019f1393-d680-75df-96d6-3eddbd1735fc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019f1393-d680-7bf5-a113-e3033b11b221" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019f1393-d680-75df-96d6-3eddbd1735fc" xlink:to="loc_srt_MinimumMember_019f1393-d680-7bf5-a113-e3033b11b221" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019f1393-d680-758e-b520-89c2948117f6" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019f1393-d680-75df-96d6-3eddbd1735fc" xlink:to="loc_srt_MaximumMember_019f1393-d680-758e-b520-89c2948117f6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019f1393-d680-7752-ad6f-ec1a18ed5948" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019f1393-d680-72e4-8a51-cfd2021b06a1" xlink:to="loc_us-gaap_AwardTypeAxis_019f1393-d680-7752-ad6f-ec1a18ed5948" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019f1393-d680-7752-ad6f-ec1a18ed5948_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019f1393-d680-7752-ad6f-ec1a18ed5948" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019f1393-d680-7752-ad6f-ec1a18ed5948_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019f1393-d680-7c18-a15a-e999d8e5fd1d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019f1393-d680-7752-ad6f-ec1a18ed5948" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019f1393-d680-7c18-a15a-e999d8e5fd1d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_019f1393-d680-7861-a24f-05d54a68bc7a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019f1393-d680-7c18-a15a-e999d8e5fd1d" xlink:to="loc_us-gaap_EmployeeStockOptionMember_019f1393-d680-7861-a24f-05d54a68bc7a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_019f1749-8e6c-7c32-aa00-c9ca007a714a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f1393-d680-7d15-a0c9-ec0e0d55c7b7" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_019f1749-8e6c-7c32-aa00-c9ca007a714a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_019f1393-d680-7877-89db-bdbdce69ab39" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f1393-d680-7d15-a0c9-ec0e0d55c7b7" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_019f1393-d680-7877-89db-bdbdce69ab39" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_019f1393-d680-76f2-ba8d-ca64e698f1cc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f1393-d680-7d15-a0c9-ec0e0d55c7b7" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_019f1393-d680-76f2-ba8d-ca64e698f1cc" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.energyfocusinc.com/role/STOCKHOLDERSEQUITYRestrictedStockUnitsandFullyVestedSharesNarrativeDetails" xlink:type="simple" xlink:href="efoi-20260630.xsd#STOCKHOLDERSEQUITYRestrictedStockUnitsandFullyVestedSharesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.energyfocusinc.com/role/STOCKHOLDERSEQUITYRestrictedStockUnitsandFullyVestedSharesNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f1393-d680-71d0-afb5-28bc2dc5df09" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019f1393-d680-758e-b315-f78bc40f28ac" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f1393-d680-71d0-afb5-28bc2dc5df09" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019f1393-d680-758e-b315-f78bc40f28ac" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019f1393-d680-770d-847d-09a91f57fdbf" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019f1393-d680-758e-b315-f78bc40f28ac" xlink:to="loc_srt_RangeAxis_019f1393-d680-770d-847d-09a91f57fdbf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019f1393-d680-770d-847d-09a91f57fdbf_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019f1393-d680-770d-847d-09a91f57fdbf" xlink:to="loc_srt_RangeMember_019f1393-d680-770d-847d-09a91f57fdbf_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019f1393-d680-74f8-befe-35d70364ca14" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019f1393-d680-770d-847d-09a91f57fdbf" xlink:to="loc_srt_RangeMember_019f1393-d680-74f8-befe-35d70364ca14" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019f1393-d680-7762-bad8-76e2714d0a00" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019f1393-d680-74f8-befe-35d70364ca14" xlink:to="loc_srt_MinimumMember_019f1393-d680-7762-bad8-76e2714d0a00" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019f1393-d680-7256-b928-42b2efda005f" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019f1393-d680-74f8-befe-35d70364ca14" xlink:to="loc_srt_MaximumMember_019f1393-d680-7256-b928-42b2efda005f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019f1393-d680-70b7-b8b9-c7b4719f9741" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019f1393-d680-758e-b315-f78bc40f28ac" xlink:to="loc_us-gaap_AwardTypeAxis_019f1393-d680-70b7-b8b9-c7b4719f9741" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019f1393-d680-70b7-b8b9-c7b4719f9741_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019f1393-d680-70b7-b8b9-c7b4719f9741" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019f1393-d680-70b7-b8b9-c7b4719f9741_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019f1393-d680-76c8-ba98-206e6bb9c0fc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019f1393-d680-70b7-b8b9-c7b4719f9741" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019f1393-d680-76c8-ba98-206e6bb9c0fc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_019f1393-d680-7a60-ae16-1ad73ffb8447" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019f1393-d680-76c8-ba98-206e6bb9c0fc" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_019f1393-d680-7a60-ae16-1ad73ffb8447" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_019f1393-d680-707d-877f-a6da394629d0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019f1393-d680-758e-b315-f78bc40f28ac" xlink:to="loc_us-gaap_PlanNameAxis_019f1393-d680-707d-877f-a6da394629d0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_019f1393-d680-707d-877f-a6da394629d0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_019f1393-d680-707d-877f-a6da394629d0" xlink:to="loc_us-gaap_PlanNameDomain_019f1393-d680-707d-877f-a6da394629d0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_019f1393-d680-7b63-8010-0cf05ed3b590" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_019f1393-d680-707d-877f-a6da394629d0" xlink:to="loc_us-gaap_PlanNameDomain_019f1393-d680-7b63-8010-0cf05ed3b590" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_Plan2020Member_019f1393-d680-717e-89db-b1dfe811e456" xlink:href="efoi-20260630.xsd#efoi_Plan2020Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_019f1393-d680-7b63-8010-0cf05ed3b590" xlink:to="loc_efoi_Plan2020Member_019f1393-d680-717e-89db-b1dfe811e456" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_019f1393-d680-7584-b5f7-297093bf1ba7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f1393-d680-71d0-afb5-28bc2dc5df09" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_019f1393-d680-7584-b5f7-297093bf1ba7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingNumber_019f1393-d680-76ef-a6b3-9e5318f10708" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f1393-d680-71d0-afb5-28bc2dc5df09" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingNumber_019f1393-d680-76ef-a6b3-9e5318f10708" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensationGross_019f174c-bb80-7d2c-8ec1-045b3bcb7d95" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensationGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f1393-d680-71d0-afb5-28bc2dc5df09" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensationGross_019f174c-bb80-7d2c-8ec1-045b3bcb7d95" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_019f174c-bb80-7bc1-bbb3-a22e635b5f33" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f1393-d680-71d0-afb5-28bc2dc5df09" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_019f174c-bb80-7bc1-bbb3-a22e635b5f33" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensationForfeited_019f174c-bb80-78d1-97d9-52c0be9a54bf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensationForfeited"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f1393-d680-71d0-afb5-28bc2dc5df09" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensationForfeited_019f174c-bb80-78d1-97d9-52c0be9a54bf" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_NetIncreaseDecreaseInCommonSharesOutstanding_019f174d-4753-79d2-b66c-94302c9505dd" xlink:href="efoi-20260630.xsd#efoi_NetIncreaseDecreaseInCommonSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f1393-d680-71d0-afb5-28bc2dc5df09" xlink:to="loc_efoi_NetIncreaseDecreaseInCommonSharesOutstanding_019f174d-4753-79d2-b66c-94302c9505dd" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.energyfocusinc.com/role/COMMITMENTSANDCONTINGENCIESDetails" xlink:type="simple" xlink:href="efoi-20260630.xsd#COMMITMENTSANDCONTINGENCIESDetails"/>
  <link:definitionLink xlink:role="http://www.energyfocusinc.com/role/COMMITMENTSANDCONTINGENCIESDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermPurchaseCommitmentLineItems_019f1393-d680-750f-bb86-09d93c650799" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermPurchaseCommitmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermPurchaseCommitmentTable_019f1393-d680-7793-b184-e7d2ff39a17b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermPurchaseCommitmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LongTermPurchaseCommitmentLineItems_019f1393-d680-750f-bb86-09d93c650799" xlink:to="loc_us-gaap_LongTermPurchaseCommitmentTable_019f1393-d680-7793-b184-e7d2ff39a17b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementScenarioAxis_019f1393-d680-7a6d-91d8-ddc51a28064e" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_StatementScenarioAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LongTermPurchaseCommitmentTable_019f1393-d680-7793-b184-e7d2ff39a17b" xlink:to="loc_srt_StatementScenarioAxis_019f1393-d680-7a6d-91d8-ddc51a28064e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScenarioUnspecifiedDomain_019f1393-d680-7a6d-91d8-ddc51a28064e_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ScenarioUnspecifiedDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementScenarioAxis_019f1393-d680-7a6d-91d8-ddc51a28064e" xlink:to="loc_srt_ScenarioUnspecifiedDomain_019f1393-d680-7a6d-91d8-ddc51a28064e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScenarioUnspecifiedDomain_019f1393-d680-73fd-b2aa-2f924c18ef9a" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ScenarioUnspecifiedDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementScenarioAxis_019f1393-d680-7a6d-91d8-ddc51a28064e" xlink:to="loc_srt_ScenarioUnspecifiedDomain_019f1393-d680-73fd-b2aa-2f924c18ef9a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScenarioForecastMember_019f1393-d680-7961-8868-4055d548f762" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ScenarioForecastMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ScenarioUnspecifiedDomain_019f1393-d680-73fd-b2aa-2f924c18ef9a" xlink:to="loc_srt_ScenarioForecastMember_019f1393-d680-7961-8868-4055d548f762" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PurchaseObligation_019f1393-d680-7186-8ab8-56644f88194b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PurchaseObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongTermPurchaseCommitmentLineItems_019f1393-d680-750f-bb86-09d93c650799" xlink:to="loc_us-gaap_PurchaseObligation_019f1393-d680-7186-8ab8-56644f88194b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermPurchaseCommitmentAmount_019f1393-d680-7317-9c20-7ce60eb44ee9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermPurchaseCommitmentAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongTermPurchaseCommitmentLineItems_019f1393-d680-750f-bb86-09d93c650799" xlink:to="loc_us-gaap_LongTermPurchaseCommitmentAmount_019f1393-d680-7317-9c20-7ce60eb44ee9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_PurchaseObligationPercentage_019f1393-d680-7fca-a1e2-26af8c41aba0" xlink:href="efoi-20260630.xsd#efoi_PurchaseObligationPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongTermPurchaseCommitmentLineItems_019f1393-d680-750f-bb86-09d93c650799" xlink:to="loc_efoi_PurchaseObligationPercentage_019f1393-d680-7fca-a1e2-26af8c41aba0" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.energyfocusinc.com/role/RELATEDPARTYTRANSACTIONSScheduleofRelatedPartyTransactionsDetails" xlink:type="simple" xlink:href="efoi-20260630.xsd#RELATEDPARTYTRANSACTIONSScheduleofRelatedPartyTransactionsDetails"/>
  <link:definitionLink xlink:role="http://www.energyfocusinc.com/role/RELATEDPARTYTRANSACTIONSScheduleofRelatedPartyTransactionsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionLineItems_019f1393-d680-73e0-8f42-d8634fdf0470" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyTransactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019f1393-d680-73d2-9f3a-d20d5256dff4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019f1393-d680-73e0-8f42-d8634fdf0470" xlink:to="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019f1393-d680-73d2-9f3a-d20d5256dff4" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis_019f1393-d680-72be-9d2b-677b993248ea" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019f1393-d680-73d2-9f3a-d20d5256dff4" xlink:to="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis_019f1393-d680-72be-9d2b-677b993248ea" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain_019f1393-d680-72be-9d2b-677b993248ea_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis_019f1393-d680-72be-9d2b-677b993248ea" xlink:to="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain_019f1393-d680-72be-9d2b-677b993248ea_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain_019f1393-d680-7be0-81bc-df17b7ccd9f6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis_019f1393-d680-72be-9d2b-677b993248ea" xlink:to="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain_019f1393-d680-7be0-81bc-df17b7ccd9f6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_WinnerTechnologyInternationalLimitedAndSanderElectronicsMember_019f7eb0-c7c3-78ba-b929-54303ba84f11" xlink:href="efoi-20260630.xsd#efoi_WinnerTechnologyInternationalLimitedAndSanderElectronicsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain_019f1393-d680-7be0-81bc-df17b7ccd9f6" xlink:to="loc_efoi_WinnerTechnologyInternationalLimitedAndSanderElectronicsMember_019f7eb0-c7c3-78ba-b929-54303ba84f11" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_WinnerTechnologyInternationalLimitedMember_019f7eb0-c7c3-7c3d-adbb-a551f737b1d7" xlink:href="efoi-20260630.xsd#efoi_WinnerTechnologyInternationalLimitedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_efoi_WinnerTechnologyInternationalLimitedAndSanderElectronicsMember_019f7eb0-c7c3-78ba-b929-54303ba84f11" xlink:to="loc_efoi_WinnerTechnologyInternationalLimitedMember_019f7eb0-c7c3-7c3d-adbb-a551f737b1d7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_SanderElectronicsIncMember_019f1393-d680-7b25-8e84-4ff6f0510a39" xlink:href="efoi-20260630.xsd#efoi_SanderElectronicsIncMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_efoi_WinnerTechnologyInternationalLimitedAndSanderElectronicsMember_019f7eb0-c7c3-78ba-b929-54303ba84f11" xlink:to="loc_efoi_SanderElectronicsIncMember_019f1393-d680-7b25-8e84-4ff6f0510a39" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_SanderElectronicsCoLtdMember_019f1393-d680-7dd8-bc20-e0c5b5ec6135" xlink:href="efoi-20260630.xsd#efoi_SanderElectronicsCoLtdMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_efoi_WinnerTechnologyInternationalLimitedAndSanderElectronicsMember_019f7eb0-c7c3-78ba-b929-54303ba84f11" xlink:to="loc_efoi_SanderElectronicsCoLtdMember_019f1393-d680-7dd8-bc20-e0c5b5ec6135" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019f1393-d680-74e5-8776-d022eccc772d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskByBenchmarkAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019f1393-d680-73d2-9f3a-d20d5256dff4" xlink:to="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019f1393-d680-74e5-8776-d022eccc772d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019f1393-d680-74e5-8776-d022eccc772d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019f1393-d680-74e5-8776-d022eccc772d" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019f1393-d680-74e5-8776-d022eccc772d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019f1393-d680-731d-b1db-becbf3f133b2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019f1393-d680-74e5-8776-d022eccc772d" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019f1393-d680-731d-b1db-becbf3f133b2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_TotalExpendituresMember_019f1393-d680-7bd9-85ee-03bd854eeb69" xlink:href="efoi-20260630.xsd#efoi_TotalExpendituresMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019f1393-d680-731d-b1db-becbf3f133b2" xlink:to="loc_efoi_TotalExpendituresMember_019f1393-d680-7bd9-85ee-03bd854eeb69" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsPayableMember_019f1393-d680-72b0-8738-b9fc780631fe" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccountsPayableMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019f1393-d680-731d-b1db-becbf3f133b2" xlink:to="loc_us-gaap_AccountsPayableMember_019f1393-d680-72b0-8738-b9fc780631fe" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByTypeAxis_019f1393-d680-7fb9-b204-4212e26767a2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019f1393-d680-73d2-9f3a-d20d5256dff4" xlink:to="loc_us-gaap_ConcentrationRiskByTypeAxis_019f1393-d680-7fb9-b204-4212e26767a2" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_019f1393-d680-7fb9-b204-4212e26767a2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_019f1393-d680-7fb9-b204-4212e26767a2" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_019f1393-d680-7fb9-b204-4212e26767a2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_019f1393-d680-7119-ab12-0aa58e5ca776" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_019f1393-d680-7fb9-b204-4212e26767a2" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_019f1393-d680-7119-ab12-0aa58e5ca776" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SupplierConcentrationRiskMember_019f1393-d680-7afc-ae08-86561eac556f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SupplierConcentrationRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskTypeDomain_019f1393-d680-7119-ab12-0aa58e5ca776" xlink:to="loc_us-gaap_SupplierConcentrationRiskMember_019f1393-d680-7afc-ae08-86561eac556f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionPurchasesFromRelatedParty_019f1393-d680-7f37-aa6a-6f71473500c4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyTransactionPurchasesFromRelatedParty"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019f1393-d680-73e0-8f42-d8634fdf0470" xlink:to="loc_us-gaap_RelatedPartyTransactionPurchasesFromRelatedParty_019f1393-d680-7f37-aa6a-6f71473500c4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskPercentage1_019f1393-d680-7625-aacb-2cfd8f1b722e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskPercentage1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019f1393-d680-73e0-8f42-d8634fdf0470" xlink:to="loc_us-gaap_ConcentrationRiskPercentage1_019f1393-d680-7625-aacb-2cfd8f1b722e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsPayableCurrentAndNoncurrent_019f1393-d680-7f5a-bfba-314c02cc7358" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccountsPayableCurrentAndNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019f1393-d680-73e0-8f42-d8634fdf0470" xlink:to="loc_us-gaap_AccountsPayableCurrentAndNoncurrent_019f1393-d680-7f5a-bfba-314c02cc7358" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.energyfocusinc.com/role/RELATEDPARTYTRANSACTIONSNarrativeDetails" xlink:type="simple" xlink:href="efoi-20260630.xsd#RELATEDPARTYTRANSACTIONSNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.energyfocusinc.com/role/RELATEDPARTYTRANSACTIONSNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionLineItems_019f1756-24e6-7419-8461-289e79bbee75" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyTransactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019f1756-24e5-73c3-86f7-b690fb1fe6d8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019f1756-24e6-7419-8461-289e79bbee75" xlink:to="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019f1756-24e5-73c3-86f7-b690fb1fe6d8" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019f1756-24e5-7f3c-9d7d-837e96c50958" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskByBenchmarkAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019f1756-24e5-73c3-86f7-b690fb1fe6d8" xlink:to="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019f1756-24e5-7f3c-9d7d-837e96c50958" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019f1756-24e5-7f3c-9d7d-837e96c50958_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019f1756-24e5-7f3c-9d7d-837e96c50958" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019f1756-24e5-7f3c-9d7d-837e96c50958_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019f1756-24e5-7d5c-bf94-39d666205e74" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019f1756-24e5-7f3c-9d7d-837e96c50958" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019f1756-24e5-7d5c-bf94-39d666205e74" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsPayableMember_019f1756-24e5-780e-8800-4f3960a4272c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccountsPayableMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019f1756-24e5-7d5c-bf94-39d666205e74" xlink:to="loc_us-gaap_AccountsPayableMember_019f1756-24e5-780e-8800-4f3960a4272c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByTypeAxis_019f1756-24e5-755f-af5e-cd83c6389f1a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019f1756-24e5-73c3-86f7-b690fb1fe6d8" xlink:to="loc_us-gaap_ConcentrationRiskByTypeAxis_019f1756-24e5-755f-af5e-cd83c6389f1a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_019f1756-24e5-755f-af5e-cd83c6389f1a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_019f1756-24e5-755f-af5e-cd83c6389f1a" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_019f1756-24e5-755f-af5e-cd83c6389f1a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_019f1756-24e5-7c3d-b01d-27885d16f65e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_019f1756-24e5-755f-af5e-cd83c6389f1a" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_019f1756-24e5-7c3d-b01d-27885d16f65e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SupplierConcentrationRiskMember_019f1756-24e5-7bbe-a6bd-d4043a1a7855" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SupplierConcentrationRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskTypeDomain_019f1756-24e5-7c3d-b01d-27885d16f65e" xlink:to="loc_us-gaap_SupplierConcentrationRiskMember_019f1756-24e5-7bbe-a6bd-d4043a1a7855" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis_019f1756-24e5-72f8-b409-695f6157ce76" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019f1756-24e5-73c3-86f7-b690fb1fe6d8" xlink:to="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis_019f1756-24e5-72f8-b409-695f6157ce76" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain_019f1756-24e5-72f8-b409-695f6157ce76_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis_019f1756-24e5-72f8-b409-695f6157ce76" xlink:to="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain_019f1756-24e5-72f8-b409-695f6157ce76_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain_019f1756-24e5-7d60-a145-fce4a08fdc34" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis_019f1756-24e5-72f8-b409-695f6157ce76" xlink:to="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain_019f1756-24e5-7d60-a145-fce4a08fdc34" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_OffshoreSupplierBAndCMember_019f1756-24e5-73b2-8532-01756381ca90" xlink:href="efoi-20260630.xsd#efoi_OffshoreSupplierBAndCMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain_019f1756-24e5-7d60-a145-fce4a08fdc34" xlink:to="loc_efoi_OffshoreSupplierBAndCMember_019f1756-24e5-73b2-8532-01756381ca90" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskPercentage1_019f1756-24e6-7dc1-81fb-2130f645c34b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskPercentage1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019f1756-24e6-7419-8461-289e79bbee75" xlink:to="loc_us-gaap_ConcentrationRiskPercentage1_019f1756-24e6-7dc1-81fb-2130f645c34b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_PurchaseObligationPercentage_019f1756-24e6-7325-8503-f6af3746ebfc" xlink:href="efoi-20260630.xsd#efoi_PurchaseObligationPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019f1756-24e6-7419-8461-289e79bbee75" xlink:to="loc_efoi_PurchaseObligationPercentage_019f1756-24e6-7325-8503-f6af3746ebfc" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.energyfocusinc.com/role/SubsequentEventsDetails" xlink:type="simple" xlink:href="efoi-20260630.xsd#SubsequentEventsDetails"/>
  <link:definitionLink xlink:role="http://www.energyfocusinc.com/role/SubsequentEventsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventLineItems_019fa7a2-9dd5-7666-8394-5d7ff2932d1a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTable_019f1393-d680-7388-b014-a2545577fc7f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SubsequentEventLineItems_019fa7a2-9dd5-7666-8394-5d7ff2932d1a" xlink:to="loc_us-gaap_SubsequentEventTable_019f1393-d680-7388-b014-a2545577fc7f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_019fa7a2-9dd5-7354-8875-890b89e05ec8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_019f1393-d680-7388-b014-a2545577fc7f" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_019fa7a2-9dd5-7354-8875-890b89e05ec8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_019fa7a2-9dd5-7354-8875-890b89e05ec8_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_019fa7a2-9dd5-7354-8875-890b89e05ec8" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_019fa7a2-9dd5-7354-8875-890b89e05ec8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_019fa7a2-9dd5-73be-af11-705adacbd19f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_019fa7a2-9dd5-7354-8875-890b89e05ec8" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_019fa7a2-9dd5-73be-af11-705adacbd19f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventMember_019fa7a2-9dd5-738f-ae24-ce81ecad638d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_019fa7a2-9dd5-73be-af11-705adacbd19f" xlink:to="loc_us-gaap_SubsequentEventMember_019fa7a2-9dd5-738f-ae24-ce81ecad638d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainContingenciesByNatureAxis_019fc87a-4248-77c8-8c55-5fde88e8d1e1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_GainContingenciesByNatureAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_019f1393-d680-7388-b014-a2545577fc7f" xlink:to="loc_us-gaap_GainContingenciesByNatureAxis_019fc87a-4248-77c8-8c55-5fde88e8d1e1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainContingencyNatureDomain_019fc87a-4248-77c8-8c55-5fde88e8d1e1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_GainContingencyNatureDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_GainContingenciesByNatureAxis_019fc87a-4248-77c8-8c55-5fde88e8d1e1" xlink:to="loc_us-gaap_GainContingencyNatureDomain_019fc87a-4248-77c8-8c55-5fde88e8d1e1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainContingencyNatureDomain_019fc87a-4248-7fa1-8e4b-d629358997a3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_GainContingencyNatureDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_GainContingenciesByNatureAxis_019fc87a-4248-77c8-8c55-5fde88e8d1e1" xlink:to="loc_us-gaap_GainContingencyNatureDomain_019fc87a-4248-7fa1-8e4b-d629358997a3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_efoi_IEEPATariffRefundMember_019fc87a-4248-76ac-87c2-039c6ee39838" xlink:href="efoi-20260630.xsd#efoi_IEEPATariffRefundMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GainContingencyNatureDomain_019fc87a-4248-7fa1-8e4b-d629358997a3" xlink:to="loc_efoi_IEEPATariffRefundMember_019fc87a-4248-76ac-87c2-039c6ee39838" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RecoveryOfDirectCosts_019fc87a-4248-76eb-baf5-025d5ad0b9dc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RecoveryOfDirectCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_019fa7a2-9dd5-7666-8394-5d7ff2932d1a" xlink:to="loc_us-gaap_RecoveryOfDirectCosts_019fc87a-4248-76eb-baf5-025d5ad0b9dc" xlink:type="arc" order="0"/>
  </link:definitionLink>
</link:linkbase>
