The Collaborative International Dictionary of English v.0.48:

Gulf \Gulf\ (g[u^]lf), n. [F. golfe, It. golfo, fr. Gr. ko`lpos
   bosom, bay, gulf, LGr. ko`lfos.]
   1. A hollow place in the earth; an abyss; a deep chasm or
      basin,
      [1913 Webster]

            He then surveyed
            Hell and the gulf between.            --Milton.
      [1913 Webster]

            Between us and you there is a great gulf fixed.
                                                  --Luke xvi.
                                                  26.
      [1913 Webster]

   2. That which swallows; the gullet. [Obs.] --Shak.
      [1913 Webster]

   3. That which swallows irretrievably; a whirlpool; a sucking
      eddy. --Shak.
      [1913 Webster]

            A gulf of ruin, swallowing gold.      --Tennyson.
      [1913 Webster]

   4. (Geog.) A portion of an ocean or sea extending into the
      land; a partially land-locked sea; as, the Gulf of Mexico.
      [1913 Webster]

   5. (Mining) A large deposit of ore in a lode.
      [1913 Webster]

   Gulf Stream (Geog.), the warm ocean current of the North
      Atlantic.

   Note: It originates in the westward equatorial current, due
         to the trade winds, is deflected northward by Cape St.
         Roque through the Gulf of Mexico, and flows parallel to
         the coast of North America, turning eastward off the
         island of Nantucket. Its average rate of flow is said
         to be about two miles an hour. The similar Japan
         current, or Kuro-Siwo, is sometimes called the Gulf
         Stream of the Pacific.

   Gulf weed (Bot.), a branching seaweed (Sargassum
      bacciferum, or sea grape), having numerous berrylike air
      vessels, -- found in the Gulf Stream, in the Sargasso Sea,
      and elsewhere.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

gulfweed \gulf"weed`\ (g[u^]lf"w[=e]d`), n.
   1. A brown seaweed (Sargassum bacciferum) with rounded
      bladders forming dense floating masses in tropical
      Atlantic waters as in the Sargasso Sea.

   Syn: sargassum, sargasso, Sargassum bacciferum.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

Sargassum bacciferum
    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]