The Collaborative International Dictionary of English v.0.48:

helianthus \helianthus\ n.
   any plant of the genus Helianthus having large flower heads
   with dark disk florets and showy yellow rays.

   Syn: sunflower.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

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