A sliding piece which either is moved by, or moves against, fluid pressure. It usually consists of a short cylinder fitting within a cylindrical vessel along which it moves, back and forth. It is used in steam engines to receive motion from the steam, and in pumps to transmit motion to a fluid; also for other purposes.
- A solid cylinder or disk that fits snugly into a larger cylinder and moves under fluid pressure, as in a reciprocating engine, or displaces or compresses fluids, as in pumps and compressors.
- Music A valve mechanism in brass instruments for altering the pitch.