The Collaborative International Dictionary of English v.0.48:

Kelp \Kelp\ (k[e^]lp), n. [Formerly kilpe; of unknown origin.]
   1. The calcined ashes of seaweed, -- formerly much used in
      the manufacture of glass, now used in the manufacture of
      iodine.
      [1913 Webster]

   2. (Bot.) Any large blackish seaweed.
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   Note: Laminaria is the common kelp of Great Britain;
         Macrocystis pyrifera and Nereocystis Lutkeana are
         the great kelps of the Pacific Ocean.
         [1913 Webster]

   Kelp crab (Zool.), a California spider crab (Epialtus
      productus), found among seaweeds, which it resembles in
      color.

   Kelp salmon (Zool.), a serranoid food fish (Serranus
      clathratus) of California. See Cabrilla.
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The Collaborative International Dictionary of English v.0.48:

Macrocystis \Mac`ro*cys"tis\, n. [NL. See Macro-, and Cyst.]
   (Bot.)
   An immensely long blackish seaweed of the Pacific
   (Macrocystis pyrifera), having numerous almond-shaped air
   vessels.
   [1913 Webster]