Emily Dickinson Lexicon

PIS'TON, n. [Fr. and Sp. piston, from the root of Sp. pisar, pistar. L. pinso, the primary sense of which is to press, send, drive, thrust or strike, like embolus, from Gr. εμβαλλω, βαλλω.]

A short cylinder of metal or other solid substance, used in pumps and other engines or machines for various purposes. It is fitted exactly to the bore of another body so as to prevent the entrance or escape of air, and is usually applied to the purpose of forcing some fluid into or out of the canal or tube which it fills, as in pumps, fire-engines and the like.