wallflower

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of various plants of the genus Erysimum of the mustard family, having fragrant yellow, orange, or brownish flowers, especially the widely cultivated species E. cheiri.
• noun One who does not participate in the activity at a social event because of shyness or unpopularity.
• noun A security, company, or industry that is out of favor with investors.

from The Century Dictionary.

• noun An old favorite garden flower and pot-plant, Cheiranthus Cheiri, native in southern Europe, where it grows on old walls, cliffs, and the sides of quarries.
• noun A man or woman who, at a ball or party, sits by the wall, or looks on without dancing, either from choice or from being unable to dance or to obtain a partner.

from the GNU version of the Collaborative International Dictionary of English.

• noun (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.
• noun colloq. A lady at a ball, who, either from choice, or because not asked to dance, remains a spectator.
• noun (Bot.) In Australia, the desert poison bush (Gastrolobium grandiflorum); -- called also native wallflower.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Several short-lived herbs or shrubs of the Erysimum genus with bright yellow to red flowers.
• noun informal A person who is socially awkward, especially one who does not dance at a party due to shyness.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun 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
• noun any of numerous plants of the genus Erysimum having fragrant yellow or orange or brownish flowers
• noun remains on sidelines at social event