Piston

n.
  1. A sliding piece which either is moved by, or moves against, fluid pressure. It usually consists of a short cylinder fitting within a cylindrical vessel along which it moves, back and forth. It is used in steam engines to receive motion from the steam, and in pumps to transmit motion to a fluid; also for other purposes.
    2. Piston head (Steam Eng.),
    that part of a piston which is made fast to the piston rod.
    Piston rod ,
    a rod by which a piston is moved, or by which it communicates motion.
    Piston valve (Steam Eng.),
    a slide valve, consisting of a piston, or connected pistons, working in a cylindrical case which is provided with ports that are traversed by the valve.