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.