thermal reservoir

In physics, a thermal reservoir is a system that can exchange heat with another system and maintain a constant temperature. This is a fundamental concept in the second law of thermodynamics, which governs the direction of heat flow and the efficiency of energy conversion.1

A thermal reservoir is essentially a large body of matter that can absorb or release significant amounts of heat without changing its own temperature. This property makes it useful for defining thermodynamic processes. For example, when heat flows from a hotter reservoir to a colder one, the total entropy of the universe increases, which is consistent with the second law of thermodynamics.1

While the term "reservoir" can also refer to water storage areas, in physics, it specifically denotes a system with a constant temperature capable of heat exchange.2