The Collaborative International Dictionary of English v.0.48:

Ragweed \Rag"weed`\ (r[a^]g"w[=e]d`), n. (Bot.)
   A common American composite weed (Ambrosia artemisiaefolia)
   with finely divided leaves; hogweed.
   [1913 Webster]

   Great ragweed, a coarse American herb (Ambrosia trifida),
      with rough three-lobed opposite leaves.
      [1913 Webster]

WordNet (r) 3.0 (2006):

ragweed
    n 1: widespread European weed having yellow daisylike flowers;
         sometimes an obnoxious weed and toxic to cattle if consumed
         in quantity [syn: ragwort, tansy ragwort, ragweed,
         benweed, Senecio jacobaea]
    2: any of numerous chiefly North American weedy plants
       constituting the genus Ambrosia that produce highly
       allergenic pollen responsible for much hay fever and asthma
       [syn: ragweed, ambrosia, bitterweed]