Definition of: piston

(pis′tən) noun
1. Mech. A disk fitted to slide in a cylinder, as in a steam engine, and connected with a rod for receiving the pressure of or exerting pressure upon a fluid in the cylinder.
2. A valve in a wind instrument for altering the pitch of tones. [<F <Ital. pistone a piston, var. of pestone a large pestle <pestare pound <LL pistare, freq. of L pinsere pound]
STEAM–ENGINE PISTON
a. Piston, b. Cylinder. c. Piston rod.