ragweed

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Any of various weeds of the genus Ambrosia having small, greenish, unisexual flower heads and producing abundant pollen that is one of the chief causes of hay fever.
2. n. Chiefly British Ragwort.

Century Dictionary and Cyclopedia

1. n. Any plant of the composite genus Ambrosia; especially, the common North American species A. trifida, the great ragweed or horse-cane, and A. artemisiæfolia, the Roman wormwood or hogweed. Both are sometimes called bitterweed. The former is commonly found on river-banks, has three-lobed leaves, and is sometimes 12 feet high. The latter, a much-branching plant from 1 to 3 feet high, with dissected leaves, grows everywhere in waste places, along roads, etc., and is troublesome in fields. Its pollen is regarded as a cause of hay-fever. The plants of this genus are monœcious, the flowers of the two sexes borne in separate heads, the female heads producing a single flower with the ovoid involucre closed over it. The flowers are greenish and inconspicuous. See Ambrosia, 2.
2. n. The ragwort or St.-James-wort, Senecio Jacobæa.

Wiktionary

1. n. A plant of the genus Ambrosia. These weeds are particularly noted for producing pollen which people with hay fever are allergic to.

GNU Webster's 1913

1. n. (Bot.) A common American composite weed (Ambrosia artemisiæfolia) with finely divided leaves; hogweed.

WordNet 3.0

1. n. any of numerous chiefly North American weedy plants constituting the genus Ambrosia that produce highly allergenic pollen responsible for much hay fever and asthma
2. n. widespread European weed having yellow daisylike flowers; sometimes an obnoxious weed and toxic to cattle if consumed in quantity