Real Dictionary

ragweed
uncountable noun: Ragweed is a plant, found mainly in North America, that produces large amounts of pollen, which causes hay fever.

ragweed in American English
any of a genus (Ambrosia) of chiefly North American plants of the composite family, having tassel-like, greenish flowers which yield large amounts of wind-borne pollen, a major cause of hay fever
noun: any of the composite plants of the genus Ambrosia, the airborne pollen of which is the most prevalent cause of autumnal hay fever, as the common North American species, A. trifida (great ragweed or giant ragweed) and A. artemisiifolia

ragweed in British English
noun: any plant of the chiefly North American genus Ambrosia, such as A. artemisiifolia (common ragweed): family Asteraceae (composites). Their green tassel-like flowers produce large amounts of pollen, which causes hay fever