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]