The Collaborative International Dictionary of English v.0.48:

Valance \Val"ance\, n. [Perhaps fr. OF. avalant descending,
   hanging down, p. pr. of avaler to go down, let down, descend
   (cf. Avalanche); but probably from the town of Valence in
   France.]
   [1913 Webster]
   1. Hanging drapery for a bed, couch, window, or the like,
      especially that which hangs around a bedstead, from the
      bed to the floor. [Written also valence.]
      [1913 Webster]

            Valance of Venice gold in needlework. --Shak.
      [1913 Webster]

   2. The drooping edging of the lid of a trunk, which covers
      the joint when the lid is closed.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Valence \Va"lence\, n. [From L. valens, -entis, p. pr. of valere
   to have power, to be strong. See Valiant.] (Chem.)
   The degree of combining power of an atom (or radical) as
   shown by the number of atoms of hydrogen (or of other monads,
   as chlorine, sodium, etc.) with which it will combine, or for
   which it can be substituted, or with which it can be
   compared; thus, an atom of hydrogen is a monad, and has a
   valence of one; the atoms of oxygen, nitrogen, and carbon are
   respectively dyads, triads, and tetrads, and have a valence
   respectively of two, three, and four.
   [1913 Webster]

   Note: The valence of certain elements varies in different
         compounds. Valence in degree may extend as high as
         seven or eight, as in the cases of iodine and osmium
         respectively. The doctrine of valence has been of
         fundamental importance in distinguishing the
         equivalence from the atomic weight, and is an essential
         factor in explaining the chemical structures of
         compounds.
         [1913 Webster]

WordNet (r) 3.0 (2006):

valence
    n 1: (biology) a relative capacity to unite or react or interact
         as with antigens or a biological substrate [syn: valence,
         valency]
    2: (chemistry) a property of atoms or radicals; their combining
       power given in terms of the number of hydrogen atoms (or the
       equivalent) [syn: valence, valency]