The Collaborative International Dictionary of English v.0.48:

Wallflower \Wall"flow`er\, n.
   1. (Bot.) A perennial, cruciferous plant (Cheiranthus
      Cheiri), with sweet-scented flowers varying in color from
      yellow to orange and deep red. In Europe it very common on
      old walls.
      [1913 Webster]

   Note: The name is sometimes extended to other species of
         Cheiranthus and of the related genus Erysimum,
         especially the American Western wallflower (Erysimum
         asperum), a biennial herb with orange-yellow flowers.
         [1913 Webster]

   2. A lady at a ball, who, either from choice, or because not
      asked to dance, remains a spectator. [Colloq.]
      [1913 Webster]

   3. (Bot.) In Australia, the desert poison bush (Gastrolobium
      grandiflorum); -- called also native wallflower.
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

wallflower
    n 1: any of numerous plants of the genus Erysimum having
         fragrant yellow or orange or brownish flowers
    2: perennial of southern Europe having clusters of fragrant
       flowers of all colors especially yellow and orange; often
       naturalized on old walls or cliffs; sometimes placed in genus
       Erysimum [syn: wallflower, Cheiranthus cheiri, Erysimum
       cheiri]
    3: remains on sidelines at social event