A piston valve is a type of valve used to control the flow and direction of fluids such as steam, air, or liquids in mechanical and industrial systems. It consists of a solid cylindrical piston that moves back and forth inside a precisely machined cylindrical chamber. As the piston reciprocates, it alternately opens and closes inlet and outlet ports, allowing fluid to enter, exit, or change direction at the correct time. This motion is usually driven by a crankshaft or cam mechanism in engines.