thermal reservoir
thermodynamic system with a heat capacity that is large enough that when it is in thermal contact with another system of interest or its environment, its temperature remains effectively constant
en-GB/ˈθɜːml ˈɹɛzəvwɑːɹ/
en-US/ˈθɝmæl ˈɹɛzɚˌvwæɹ/