In thermodynamics, a (hypothetical) body with a large thermal energy capacity that can absorb or supply finite amounts of heat without undergoing a change in temperature.
A thermal energy reservoir can be as large as the atmosphere, or as small as the inside of a refrigerator, as long as the reservoir remains at a constant temperature during the course of the heat exchange being monitored.
A reservoir that supplies heat energy - say, the sun, a furnace, or a person in a small room - is called a source.
A reservoir that absorbs energy - say, the room surrounding a refrigerator, the atmosphere around an incoming spaceship, or the body of a person who swallows a hot drink - is called a sink.
Thermal reservoirs are much encountered in the study of heat engines and other forms of heat exchange.