Source : Webster's Revised Unabridged Dictionary (1913)

Sunflower \Sun"flow`er\, n.
   Any plant of the genus {Helianthus}; -- so called probably
   from the form and color of its flower, which is large disk
   with yellow rays. The commonly cultivated sunflower is
   {Helianthus annuus}, a native of America.

Source : WordNet®

Helianthus annuus
     n : annual sunflower grown for silage and for its seeds which
         are a source of oil; common throughout United States and
         much of North America [syn: {common sunflower}, {mirasol}]