Source : Webster's Revised Unabridged Dictionary (1913)

Ragweed \Rag"weed\, n. (Bot.)
   A common American composite weed ({Ambrosia
   artemisi[ae]folia}) with finely divided leaves; hogweed.

   {Great ragweed}, a coarse American herb ({Ambrosia trifida}),
      with rough three-lobed opposite leaves.

Source : WordNet®

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}, {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: {ambrosia}, {bitterweed}]