A system that exchanges energy but not matter with its surroundings is called a closed system. Its boundary prevents mass from entering or leaving, but heat and work can still cross the boundary.
A sealed container being heated is a simple example: the container’s contents remain inside, while energy enters as heat. A piston-cylinder can also represent a closed system when the gas is trapped but the piston allows work transfer. In chemical thermodynamics, reactions may change which molecules are present, but the total amount of each chemical element remains conserved within the sealed boundary.
The most frequent confusion is with an isolated system. An isolated system exchanges neither matter nor energy, whereas a closed system can exchange energy. An open system allows both matter and energy to cross. “Adiabatic” narrows the description in another direction: it means no heat transfer, but an adiabatic system could still exchange work or, if open, mass. These labels depend on what crosses the chosen system boundary.