The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sunflower
    n 1: any plant of the genus Helianthus having large flower heads
         with dark disk florets and showy yellow rays [syn:
         sunflower, helianthus]