Source : Webster's Revised Unabridged Dictionary (1913)

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.

   {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.