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