any plant of the genus Helianthus having large flower heads with dark disk florets and showy yellow rays
"The gardener carefully selected a sunflower for the border because its massive head featured deep brown disks surrounded by brilliant yellow petals."
Any plant of the genus Helianthus, so called probably from the form and color of its floral head, having the form of a large disk surrounded by yellow ray flowers.
"The gardener carefully transplanted the young sunflower to the south side of the garden where it would get enough direct sunlight for its large, disk-shaped heads to fully bloom in bright yellow."