Real Dictionary

sunflower
countable noun: A sunflower is a very tall plant with large yellow flowers. Oil from sunflower seeds is used in cooking and to make margarine.

sunflower in American English
any of a genus (Helianthus) of tall plants of the composite family, having large, yellow, daisylike flowers with yellow, brown, purple, or almost black disks containing edible seeds that yield an oil
noun: any of various composite plants of the genus Helianthus, as H. annuus, having showy, yellow-rayed flower heads often 12 in. (30 cm) wide, and edible seeds that yield an oil with a wide variety of uses: the state flower of Kansas

sunflower in British English
noun: any of several American plants of the genus Helianthus, esp H. annuus, having very tall thick stems, large flower heads with yellow rays, and seeds used as food, esp for poultry: family Asteraceae (composites)