High Quality Content by WIKIPEDIA articles! In thermodynamics, a heat reservoir, thermal reservoir, or heat bath is a system whose heat capacity is so large that when it is in thermal contact with some other system of interest its temperature remains effectively constant. The heat bath is effectively an infinite reservoir of energy and accessible quantum states at a given temperature. It is considered to be a constant temperature source. The temperature of the reservoir does not change irrespective of whether heat is added or extracted to or from it.