piston

A central component in many engines and mechanisms, a piston is short, cylindrical part that moves up and down inside a fitting tube as a result of expanding gas or liquid, usually the result of an applied heating process, such as combustion. As the gas or liquid expands, forcing the piston through its cylinder, the piston transfers its movement to other parts of the mechanism via a rod that connects the piston to a crankshaft or flywheel.