Heat reservoir
An idealised body so large that it can absorb or supply heat without its own temperature changing — the constant-temperature partner of an isothermal process.
Definition
A heat reservoir (or thermal reservoir) is an idealised body with so large a heat capacity that any amount of heat it gives up or takes in leaves its temperature effectively unchanged. It is the thermodynamic equivalent of an infinite bath: a system placed in contact with it can exchange heat freely while the reservoir holds a fixed temperature, the precondition for a genuinely isothermal process.
Reservoirs are the terminals of heat engines and refrigerators. An engine draws heat from a hot reservoir, converts part of it to work, and rejects the rest to a cold reservoir; a refrigerator runs the same arrangement in reverse, using work to move heat from cold to hot. The temperatures of the two reservoirs set the ceiling on an engine's efficiency through the Carnot limit.
Real reservoirs are approximations — the ocean, the atmosphere, or a large water bath act as reservoirs over short experiments. The idealisation is indispensable for defining isothermal processes and for the reservoir-to-reservoir framing in which the second law is most naturally stated.
History
Formalised in Carnot's 1824 picture of an engine working between a hot 'source' and cold 'sink', and made central to the second law by Kelvin and Clausius, whose statements of the law are framed in terms of heat exchanged with reservoirs.