n : any of various chiefly No. American weedy herbaceous plants comprising the genus Ambrosia and producing highly allergenic pollen responsible for much hay fever and asthma: as a : an annual weed (A. artemisiifolia) with finely divided foliage that is common on open or cultivated ground in much of No. America b : a coarse annual (A. trifida) with some or all of the leaves usu. deeply and palmately 3-cleft or 5-cleft - called also great ragweed .
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