The Collaborative International Dictionary of English v.0.48:

Multivalent \Mul*tiv"a*lent\, a. [Multi- + L. valens, p. pr. See
   Valence.]
   1. (Chem.)
      (a) Having a valence greater than one, as silicon.
      (b) Having more than one degree of valence, as sulphur.
          [1913 Webster]

   2. (Immunology) Having multiple antigen-binding sites; -- of
      antibody molecules.
      [PJC]

   3. (Immunology) Containing several types of antibody, to
      protect against more than one disease; polyvalent; -- of
      vaccines.
      [PJC]

WordNet (r) 3.0 (2006):

multivalent
    adj 1: used of the association of three or more homologous
           chromosomes during the first division of meiosis [ant:
           bivalent, double, univalent]
    2: having more than one valence, or having a valence of 3 or
       higher [syn: polyvalent, multivalent] [ant: monovalent,
       univalent]
    3: having many values, meanings, or appeals; "subtle,
       multivalent allegory" [syn: multivalent, multi-valued]