\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.