The Collaborative International Dictionary of English v.0.48:

Sargassum \Sar*gas"sum\, n. [NL.]
   A genus of algae including the gulf weed.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sargassum
    n 1: brown algae with rounded bladders forming dense floating
         masses in tropical Atlantic waters as in the Sargasso Sea
         [syn: gulfweed, sargassum, sargasso, Sargassum
         bacciferum]