The Collaborative International Dictionary of English v.0.48:

Piston \Pis"ton\, n. [F. piston; cf. It. pistone piston, also
   pestone a large pestle; all fr. L. pinsere, pistum, to pound,
   to stamp. See Pestle, Pistil.] (Mach.)
   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.
   [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Piston
    n 1: United States neoclassical composer (1894-1976) [syn:
         Piston, Walter Piston]
    2: mechanical device that has a plunging or thrusting motion
       [syn: piston, plunger]