valve

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of various devices that regulate the flow of gases, liquids, or loose materials through piping or through apertures by opening, closing, or obstructing ports or passageways.
• noun The movable control element of such a device.
• noun A device in a brass wind instrument that can be opened or closed to change the pitch by altering the length of the air column in the tube.
• noun Anatomy A membranous structure in a hollow organ or passage, as in an artery or vein, that folds or closes to prevent the return flow of the body fluid passing through it.
• noun A piece of shell covering or enclosing certain mollusks or other invertebrates, especially the single one of a univalve mollusk or one of the paired hinged ones of a bivalve mollusk or brachiopod.
• noun One of the two siliceous halves of the cell wall of a diatom.
• noun One of the pieces into which a plant part splits at maturity, especially a segment of a fruit capsule or of certain anthers.
• noun Chiefly British An electron tube or a vacuum tube.
• noun Archaic Either half of a double or folding door.
• transitive verb To provide with a valve.
• transitive verb To control by means of a valve.

from The Century Dictionary.

• noun metal arcs, especially the mercury arc.
• noun A lifting-valve in which the contact with the top surface above the opening in the seat has a plane or flat area, as distinguished from one in which a part of the surface of a cone fits into a conical edge in the opening.
• noun A clack- or flap-valve.
• noun Specifically, in an internal-combustion motor cylinder, the valve through which the combustible mixture of fuel and air enters, having been previously proportioned by proper mixing apparatus, usually also valves.
• noun A cylindrical valve, fitting a cylindrical casing, controlling openings or ports made in the cylindrical surface by a rotation of the cylinder around its axis. When the valve is a cone fitting a conical surface it is a plug-valve.
• noun A valve held shut by a determined force, such as a weight or spring, so that by it the pressure in the duct or passage may be tested.
• noun apply the brake, and
• noun release the brake. Its essential elements are a balanced piston, on one side of which is the pressure in the brake-pipe and on the other side the pressure in the auxiliary reservoir; a slide-valve, on top of which is the pressure in the auxiliary reservoir; and a graduating-valve, which opens or closes certain ports in the slide-valve. When charging the system, air from the brake-pipe passes through a feed-groove around the piston of the triple valve and so into the auxiliary reservoir, charging it with compressed air, the slide-valve meanwhile connecting the brake-cylinder to the exhaust port and atmosphere through ports in its face. When the pressure in the brake-pipe falls below that in the auxiliary reservoir, the balance of pressure on the piston is destroyed and the piston moves toward the now lower brake-pipe pressure, carrying with it the graduating- and slide-valves which cut off the auxiliary reservoir from the brake-pipe
• noun cut off the brake-cylinder from the atmosphere, and
• noun connect the auxiliary reservoir with the brake-cylinder, thus admitting compressed air to the brake-cylinder, which forces the piston in this cylinder outward, and, through the connecting levers and rods, sets the brake. For any definite reduction in brake-pipe pressure short of the point at which the brake-cylinder and auxiliary reservoir pressures equalize, the increase of brake-cylinder pressure continues until the auxiliary reservoir pressure falls slightly below that remaining in the brake-pipe when the triple-valve piston returns in the direction of the now lower auxiliary reservoir pressure, carrying with it the graduating-valve, until the latter cuts off the flow of air from the auxiliary reservoir to the brake-cylinder, when all communication through the triple valve is closed. When the pressure in the brake-pipe is then increased above that in the auxiliary reservoir sufficiently to overcome the resistance of both the piston and slide-valve, the parts return to their first position, charging the auxiliary reservoir anew and connecting the brake-cylinder to the atmosphere, which releases its charge and allows the release-springs to return its piston with the attached levers, rods, and brake-shoes to their original positions and releasing the shoes from the wheels. Various improved types of triple valves are now in use, providing for differentiating between service and emergency brake-cylinder pressures, for graduating the release of all brakes in the train, for a rapid serial service application of all the brakes in long trains, for uniform releasing and uniform recharging of all the brakes in long trains, and so on.
• noun One of the leaves of a folding door; in the plural, a folding door.
• noun Any device or appliance used to control the flow of a liquid, vapor, or gas, or loose material in bulk, through a pipe, passageway, outlet, or inlet, in any form of containing vessel.
• noun In anatomy and zoology, a membranous part, fold, or thin layer which resembles a valve, or actually serves as a valve in connection with the flow of blood, lymph, or other fluid; a valva or valvular as, the valve of Vieussens in the brain; the connivent valves of Kerkring in the intestine; valves of the heart, of the veins, etc. See cuts under bulb, Crinoidea, heart, lymphatic, and vein.
• noun In botany, in flowering plants, one of the segments into which a capsule dehisces, or which opens like a lid in the dehiscence of certain anthers. In Diatomaceæ each half of the silicified membrane or shell is called a valve. See cuts under Marsilea, septicidal, and silicle.
• noun In conchology, one of the two or more separable pieces of which the shell may consist, or the whole shell when it is in one piece; each shell, right and left, of ordinary bivalves, and each shell, dorsal and ventral, of brachiopods. See bivalve, multivalve, univalve, equivalve, inequivalve, and cuts under Caprotinidæ, Chamidæ, integropalliate, and sinupalliate.
• noun In entomology, a covering plate or sheath of any organ, generally one of a pair of plates which unite to form a tube or vagina, as those covering the external sexual organs, ovipositor, etc.
• noun Any formation serving to obstruct or close the pyloric orifice of the stomach. A pylorus may have a valvular construction, or a muscular sphincter may surround the orifice. See pylorus, 2 .

from the GNU version of the Collaborative International Dictionary of English.

• noun A door; especially, one of a pair of folding doors, or one of the leaves of such a door.
• noun A lid, plug, or cover, applied to an aperture so that by its movement, as by swinging, lifting and falling, sliding, turning, or the like, it will open or close the aperture to permit or prevent passage, as of a fluid.
• noun (Anat.) One or more membranous partitions, flaps, or folds, which permit the passage of the contents of a vessel or cavity in one direction, but stop or retard the flow in the opposite direction.
• noun One of the pieces into which a capsule naturally separates when it bursts.
• noun One of the two similar portions of the shell of a diatom.
• noun A small portion of certain anthers, which opens like a trapdoor to allow the pollen to escape, as in the barberry.
• noun (Zoöl.) One of the pieces or divisions of bivalve or multivalve shells.
• noun etc. See under Air. Ball, Check, etc.
• noun a kind of balance valve usually consisting of a movable, open-ended, turban-shaped shell provided with two faces of nearly equal diameters, one above another, which rest upon two corresponding seats when the valve is closed.
• noun A valve for permitting air, steam, water, etc., to pass into or out of a chamber so as to establish or maintain equal pressure within and without.
• noun (Mach.) a chamber in which a valve works; especially (Steam Engine), the steam chest; -- called in England valve box, and valve casing. See Steam chest, under Steam.
• noun (Mach.) that part of the surface of a valve which comes in contact with the valve seat.
• noun (Steam Engine) the system of parts by which motion is given to the valve or valves for the distribution of steam in the cylinder. For an illustration of one form of valve gear, see Link motion.
• noun (Mach.) A part or piece on which such a surface is formed.
• noun (Mach.) a rod attached to a valve, for moving it.
• noun (Mach.) a strap embracing a slide valve and connecting it to the valve stem.