Source : Webster's Revised Unabridged Dictionary (1913)

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

   {Sargasso Sea}, a large tract of the North Atlantic Ocean
      where sargasso in great abundance floats on the surface.

Source : WordNet®

Sargasso Sea
     n : a vast area of the North Atlantic from the West Indies to
         the Azores that is dense with gulfweed