A piston valve is a type of valve used to control the flow of fluids such as steam, gas, or liquids in engines and pipelines. It consists of a solid cylindrical piston that slides inside a hollow cylinder. As the piston moves, it opens and closes ports, regulating the direction, pressure, and timing of flow. Piston valves are commonly used in steam engines and locomotives because they provide smoother operation, better sealing, and reduced leakage compared to other valve types.