A
thermal reservoir, a short-form of
thermal energy reservoir, or
thermal bath is a
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. It is an effectively infinite pool of
thermal energy at a given, constant
temperature. The temperature of the reservoir does not change when
heat is added or extracted because of the infinite heat capacity. As it can act as a source and sink of
heat, it is often also referred to as a
heat reservoir or
heat bath.