—n. 1. any device for halting or controlling the flow of a liquid, gas, or other material through a passage, pipe, inlet, outlet, etc.
2. a hinged lid or other movable part that closes or modifies the passage in such a device.
3. Anat.a membranous fold or other structure that controls the flow of a fluid, as one that permits blood to flow in one direction only.
4. (in musical wind instruments of the trumpet class) a device for changing the length of the air column to alter the pitch of a tone.
5. Zool. a. one of the two or more separable pieces composing certain shells.
b. either half of the silicified shell of a diatom.
6. Bot. a. one of the segments into which a capsule dehisces.
b. a flap or lidlike part of certain anthers.
7. Electronics. Chiefly Brit.See vacuum tube (def. 1).
8. Archaic.one of the leaves of a double or folding door.
—v.t. to provide with a means of controlling the flow of liquid, gas, etc., by inserting a valve.