The Collaborative International Dictionary of English v.0.48:

Sargasso \Sar*gas"so\, n. [Sp. sargazo seaweed.] (Bot.)
   The gulf weed. See under Gulf.
   [1913 Webster]

   Sargasso Sea, a large tract of the North Atlantic Ocean
      where sargasso in great abundance floats on the surface.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sargasso
    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]