In thermodynamics, closed refers to a system in which energy, in the form of work or heat, but not matter, may pass or be exchanged, through the boundary, with its surroundings. [1]

Overview
A "closed system", in short, can only have an "energy" interaction with its surroundings.

See also
Open
Isolated

References
1. Perrot, Pierre. (1998). A to Z of Thermodynamics (pg. 42). Oxford: Oxford University Press.

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