In mechanical engineering, a sliding cylinder with a closed head (the piston) that moves up and down (or back and forth) in a slightly larger cylindrical chamber (the cylinder) by or against pressure of a fluid, as in an engine or pump.
The cylinder of a steam engine is closed by plates at both ends, with provision for the piston rod, which is rigidly attached to the piston, to pass through one of the end cover plates. The cylinder of an internal-combustion engine is closed at one end by a plate called the head and open at the other end to permit free oscillation of the connecting rod, which joins the piston to the crankshaft.