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PISTON
A slidingpiece which either is moved by, or moves against, fluid pressure. It generally consists of a shortcylinder fitting within a cylindricalvessel along which it moves, back and forth. It is used in steam engines to receivemotion from the steam, and in pumps to transmitmotion to a fluid; also for another purposes. Piston head, a slide valve, consisting of a piston, or connected pistons, working in a cylindricalcase which is provided with ports that are traversed by the valve.
Origin: F. Piston; cf. It. Pistone piston, also pestone a big pestle; all fr. L. Pinsere, pistum, to pound, to stamp. See Pestle, Pistil.
Source: Websters Vocabulary