piston

Short definition from Wiktionary

Noun

1. 1
   A solid disk or cylinder that fits inside a hollow cylinder, and moves under pressure (as in an engine) or displaces fluid (as in a pump)
2. 2
   A valve device in some brass instruments for changing the pitch

Verb

1. 1
   To move up and down or in and out like a piston.

Proper noun

1. 1
   A surname.