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®

sunflower
     n : any plant of the genus Helianthus having large flower heads
         with dark disk florets and showy yellow rays [syn: {helianthus}]