In thermodynamics, open refers to a system surrounded by a boundary across which energy, in the form of work or heat, and matter, may pass or be exchanged with its surroundings. [1] In this sense, an "open system" can have any type of interaction with its surroundings.

See also
● Closed
● Isolated

References
1. Perrot, Pierre. (1998). A to Z of Thermodynamics, (pg. 214). Oxford: Oxford University Press.