Closed System
A system that can exchange energy but not matter with its surroundings.
Unit
Dimensionless
In depth
A closed system has a fixed mass, so all the change in internal energy is accounted for by heat and work crossing the boundary. Most laboratory thermodynamic measurements are designed around closed systems because the bookkeeping is simpler and more reliable.
Examples in the real world
A sealed flask warming on a hotplate; a pressure cooker; a refrigerant circuit.