piston

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. 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. n. Music A valve mechanism in brass instruments for altering the pitch.

Century Dictionary and Cyclopedia

1. n. 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. One of its sides is fitted to a rod, called the piston-rod, to which it imparts reciprocatory motion, as in the steam-engine, where the motion given to the piston-rod is communicated to the machinery, or by which, on the other hand, it is itself made to move, as in the pump. Two sorts of pistons are used in pumps —one hollow with a valve, used in the suction-pump, and the other solid, which is employed in the force-pump. The latter is also called a plunger.
2. n. 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. It is operated by depressing a finger-knob, and thus pushing a plunger into a cylinder. The plunger has channels for changing the direction of the air-column. Pistons have been applied to various instruments, but especially to the cornet, which is therefore called the cornet-à-pistons.
3. n. 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. n. 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.

Wiktionary

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

GNU Webster's 1913

1. n. (Mach.) 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.

WordNet 3.0

1. n. United States neoclassical composer (1894-1976)
2. n. mechanical device that has a plunging or thrusting motion