A closed system is a physical or conceptual environment where no matter or energy can enter or leave. In such systems, the total amount of energy remains constant, although it can change forms. An example of a closed system is a sealed container of gas, where the gas can expand or contract but cannot escape. In contrast to open systems, which exchange both matter and energy with their surroundings, closed systems are isolated from external influences. This concept is important in various fields, including thermodynamics, where it helps in understanding energy conservation and efficiency in processes like heat engines.