\Valve\, n. [L. valva the leaf, fold, or valve of a door:
cf. F. valve.]
1. A door; especially, one of a pair of folding doors, or one
   of the leaves of such a door.
         Swift through the valves the visionary fair
         Repassed.                             --Pope.
         Heavily closed, . . . the valves of the barn doors.
                                               --Longfellow.
2. 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.
Note: A valve may act automatically so as to be opened by the
      effort of a fluid to pass in one direction, and closed
      by the effort to pass in the other direction, as a
      clack valve; or it may be opened or closed by hand or
      by mechanism, as a screw valve, or a slide valve.
3. (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; as, the ileocolic, mitral,
   and semilunar valves.
4. (Bot.)
   (a) One of the pieces into which a capsule naturally
       separates when it bursts.
   (b) One of the two similar portions of the shell of a
       diatom.
   (c) A small portion of certain anthers, which opens like a
       trapdoor to allow the pollen to escape, as in the
       barberry.
5. (Zo["o]l.) One of the pieces or divisions of bivalve or
   multivalve shells.
{Air valve}, {Ball valve}, {Check valve}, etc. See under
   {Air}. {Ball}, {Check}, etc.
{Double-beat valve}, 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.
{Equilibrium valve}.
   (a) A balance valve. See under {Balance}.
   (b) 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.
{Valve chest} (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}.
{Valve face} (Mach.), that part of the surface of a valve
   which comes in contact with the {valve seat}.
{Valve gear}, or {Valve motion} (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}.
{Valve seat}. (Mach.)
   (a) The fixed surface on which a valve rests or against
       which it presses.
   (b) A part or piece on which such a surface is formed.
{Valve stem} (Mach.), a rod attached to a valve, for moving
   it.
{Valve yoke} (Mach.), a strap embracing a slide valve and
   connecting it to the valve stem.