[ 'pɪst(ə)n ]
■ noun
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 derive motion, or in a pump to impart motion.
2》 a valve in a brass instrument, depressed to alter the pitch of a note.
Origin
C18: from Fr., from Ital. ~e , var. of pestone 'large pestle'.