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):

Ambrosia trifida
    n 1: a coarse annual with some leaves deeply and palmately
         three-cleft or five-cleft [syn: great ragweed, Ambrosia
         trifida]