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 ]

. (Mach.) A spring packing ring, or any of several such rings, for a piston. [ Webster 1913 Suppl. ]