A closed system is a physical or conceptual environment where no matter can enter or leave, but energy can be exchanged with the surroundings. This means that the total mass within the system remains constant, even though energy can flow in or out. An example of a closed system is a sealed container of gas, where the gas molecules cannot escape, but heat can be added or removed. In contrast to an open system, where both matter and energy can flow in and out, a closed system allows for the study of energy transformations without the complications of mass transfer. Understanding closed systems is essential in fields like thermodynamics, where scientists analyze energy efficiency and conservation principles.