(Science: machinery) 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, 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.
Origin: F. Piston; cf. It. Pistone piston, also pestone a large pestle; all fr. L. Pinsere, pistum, to pound, to stamp. See Pestle, Pistil.