The Collaborative International Dictionary of English v.0.48:

Bivalve \Bi"valve\, n. [F. bivalve; bi- (L. bis) + valve valve.]
   1. (Zool.) A mollusk having a shell consisting of two lateral
      plates or valves joined together by an elastic ligament at
      the hinge, which is usually strengthened by prominences
      called teeth. The shell is closed by the contraction of
      two transverse muscles attached to the inner surface, as
      in the clam, -- or by one, as in the oyster. See Mollusca.
      [1913 Webster]

   2. (Bot.) A pericarp in which the seed case opens or splits
      into two parts or valves.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Bivalve \Bi"valve\, a. [Pref. bi- + valve.] (Zool. & Bot.)
   Having two shells or valves which open and shut, as the
   oyster and certain seed vessels.
   [1913 Webster]

WordNet (r) 3.0 (2006):

bivalve
    adj 1: used of mollusks having two shells (as clams etc.) [syn:
           bivalve, bivalved] [ant: univalve]
    n 1: marine or freshwater mollusks having a soft body with
         platelike gills enclosed within two shells hinged together
         [syn: bivalve, pelecypod, lamellibranch]