Constant volume refers to a condition in which the volume of a substance remains unchanged, regardless of changes in pressure or temperature. This concept is often applied in thermodynamics, where systems are analyzed under specific constraints. For example, in a closed container filled with gas, if the container's volume does not change, the gas's pressure and temperature can vary, but the overall volume remains constant. In practical applications, constant volume is crucial in processes like combustion in engines or reactions in closed systems. Understanding this principle helps scientists and engineers design efficient systems, such as internal combustion engines or pressure vessels, where maintaining a specific volume is essential for optimal performance.