The Collaborative International Dictionary of English v.0.48:

Zinnia \Zin"ni*a\, n. [NL. So called after Professor Zinn, of
   G["o]ttingen.] (Bot.)
   Any plant of the composite genus Zinnia, Mexican herbs with
   opposite leaves and large gay-colored blossoms. Zinnia
   elegans is the commonest species in cultivation.
   [1913 Webster]

WordNet (r) 3.0 (2006):

zinnia
    n 1: any of various plants of the genus Zinnia cultivated for
         their variously and brightly colored flower heads [syn:
         zinnia, old maid, old maid flower]