Piston

(pis'tun.) [Fr.] A short cylinder of metal or other solid substance, fitted exactly to the cavity of the barrel of the pump, or other machine to which it is applied. There are two kinds of pistons used in pumps, the one with a valve, and the other without one, called a plunger. The piston of the steam-engine is a circular disc, which moves up and down the cylinder; being connected by a piston-rod, which works steam tight through a stuffing-box, with the external machinery. [s. 712]