Closed system

Entities and forces that interact and are not subject to an external influence (such as the universe). A corollary of the second law of thermodynamics is that entropy grows or remains constant in a closed system. In thermodynamics, a closed system, as contrasted with an isolated system, can exchange heat and work, but not matter, with its surroundings. Here, it is assumed that a thermally isolated (insulated) system can be realized. It is a useful idealization, even if it can only be asymptotically approximated.