Definition of: valve

(valv) noun
1. Mech. Any contrivance or arrangement that permits the flow of a liquid, gas, vapor, or loose material in either of two directions, and closes against its return.
2. Obs. One of a pair of folding doors.
3. Anat. A structure formed by one or more loose folds of the lining membrane of a vessel or other organ, preventing or retarding the flow of a fluid in one direction and allowing it in another.
4. Zool. a One of the parts of a shell, as of a mollusk. b A covering plate or one of two or more external pieces forming a sheath, as for an ovipositor.
5. Bot. a One of the parts into which a capsule splits in dehiscence. b One of the halves of an anther after its opening.
6. Electr. A device for controlling the direction of flow of a current, as an electrolytic cell, or a vacuum tube.
7. Brit. A radio tube.
8. A device in certain brass-wind instruments for lengthening the air column and lowering the pitch of the instrument's scale, by turning the air current from the main tube into an additional side tube.
—v.t. valved, valv·ing To furnish with valves; control the flow of by a valve. [<L valva leaf of a door]
—valve′less adjective
GATE VALVE
Cross section.
a. Screw.
b. Gate closed.