A "closed system" refers to a physical or conceptual environment where no matter or energy can enter or leave. In such systems, all interactions occur internally, and the total amount of matter and energy remains constant. An example of a closed system is a sealed container of gas, where the gas molecules can move and interact, but no gas can escape or enter. In scientific contexts, closed systems are often used in thermodynamics to analyze energy exchanges. For instance, the first law of thermodynamics applies to closed systems, stating that energy cannot be created or destroyed, only transformed. This principle helps in understanding various processes in physics and engineering.