Wordnet 3.0
NOUN (1)
1. 
 a vast area of the North Atlantic from the West Indies to the Azores that is dense with gulfweed; 
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 Sea
    n 1: a vast area of the North Atlantic from the West Indies to
         the Azores that is dense with gulfweed