A closed system is a physical or conceptual system that does not exchange matter with its surroundings. Instead, it can exchange energy, such as heat or work. An example of a closed system is a sealed container of gas, where the gas can expand or contract but cannot escape the container. In thermodynamics, closed systems are important for understanding energy conservation and transformation. They help scientists analyze processes without the complications of external influences. The laws of thermodynamics, including the first law of thermodynamics, apply to closed systems, allowing for predictions about energy changes within the system.