- 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.
- 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
