Source : Webster's Revised Unabridged Dictionary (1913)

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.

   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.

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

Wallflower \Wall"flow`er\, n. (Bot.)
   In Australia, the desert poison bush ({Gastrolobium
   grandiflorum}); -- called also {native wallflower}.

Source : WordNet®

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: {Cheiranthus cheiri}, {Erysimum
        cheiri}]
     3: remains on sidelines at social event