The Collaborative International Dictionary of English v.0.48:

Multivalence \Mul*tiv"a*lence\, n. (Chem.)
   Quality, state, or degree, of a multivalent element, atom, or
   radical.
   [1913 Webster]

WordNet (r) 3.0 (2006):

multivalence
    n 1: (chemistry) the state of having a valence greater than two
         [syn: polyvalence, polyvalency, multivalence,
         multivalency]