ragweed

rag·weed

noun \ˈrag-ˌwēd\

Definition of RAGWEED

: any of various chiefly North 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 North America b : a coarse annual (A. trifida) with some or all of the leaves usually deeply and palmately 3-cleft or 5-cleft—called also great ragweed