n. 1 a disc or short cylinder fitting closely within a tube in which it moves up and down against a liquid or gas, used in an internal-combustion engine to impart motion, or in a pump to receive motion. 2 a sliding valve in a trumpet etc. piston-ring a ring on a piston sealing the gap between the piston and the cylinder wall. piston-rod a rod or crankshaft attached to a piston to drive a wheel or to impart motion.
[ F f. It. pistone var. of pestone augment. of pestello PESTLE ]