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.

Source: Webster's Revised Unabridged Dictionary (1913)