pis·ton / ˈpistn/ • n. a disk 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. ∎  a valve in a brass musical instrument in the form of a piston, depressed to alter the pitch of a note.

piston

piston.
1. A type of valve in brass instr., bored with passages which, when it is depressed within its casing, deflect the air-stream passing through it into the valve tubing. First valve lowers pitch a tone, 2nd a semitone, 3rd 3 semitones. If a 4th valve is added, it is adjusted to lower pitch 2 tones and a semitone, or a perfect 4th. New harmonic series can be prod. from these valves singly or in combination.

2. (Fr.). Abbreviation for cornet-à-pistons, the cornet.

piston XVIII. — F. — It. pistone, var. of pestone pestle, rammer, augm. f. pest- in pestello PESTLE.