1. 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.
2. music A valve mechanism in brass instruments for altering the pitch.

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. A valve device in some brass instruments for changing the pitch

1. 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.

1. in machinery, a movable piece, generally of a cylindrical form, so fitted as to fill the sectional area of a tube, such as the barrel of a pump or the cylinder of a steam-engine, and capable of being driven alternately in two directions by pressure on one or the other of its sides.
2. in musical wind-instruments of the trumpet family, one of the forms of valve whereby a crook is temporarily added to the tube and the pitch of the tones altered.
3. in organ-building, a thumb-knob which may be pushed in like a piston, whereby some change in registration is pneumatically effected; a pneumatic coupler or combination knob.
4. the central retractile part of the acetabulum or sucker of a cephalopod, whose action in producing a vacuum resembles that of the piston of an air-pump.

1. united states neoclassical composer (1894-1976)
2. mechanical device that has a plunging or thrusting motion