any device for halting or controlling the flow of a liquid, gas, or other material through a passage, pipe, inlet, outlet, etc.

a hinged lid or other movable part that closes or modifies the passage in such a device.

Anatomy . a membranous fold or other structure that controls the flow of a fluid, as one that permits blood to flow in one direction only.

(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.

Zoology .

one of the two or more separable pieces composing certain shells.

either half of the silicified shell of a diatom.

Botany .

one of the segments into which a capsule dehisces.

a flap or lidlike part of certain anthers.

Electronics (chiefly British ) . vacuum tube ( def. 1 ) .

Archaic . one of the leaves of a double or folding door.

to provide with a means of controlling the flow of liquid, gas, etc., by inserting a valve.

noun

cock , faucet , flap , gate , hydrant , lid , pipe , plug , shutoff , spigot , stopper , tap , outlet , regulator