A closed system is a thermodynamic system that does not exchange matter with its surroundings. This means that no mass enters or leaves the system, although energy may be exchanged. Closed systems are often contrasted with open systems, which are systems that can exchange both matter and energy with their surroundings.

Closed systems are often used to model systems that are in equilibrium, or systems that are not changing over time. For example, a closed system could be used to model a gas in a sealed container. The gas would not be able to escape from the container, and the pressure and temperature of the gas would not change over time.

Closed systems can also be used to model systems that are not in equilibrium, but that are still not exchanging matter with their surroundings. For example, a closed system could be used to model a chemical reaction. The reactants and products of the reaction would not be able to escape from the system, but the concentrations of the reactants and products would change over time.

Closed systems are an important tool for understanding the behavior of systems that are not in equilibrium. They can be used to model a variety of systems, from chemical reactions to biological processes.