Source : Webster's Revised Unabridged Dictionary (1913)

Kelp \Kelp\, 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.

   2. (Bot.) Any large blackish seaweed.

   Note: {Laminaria} is the common kelp of Great Britain;
         {Macrocystis pyrifera} and {Nereocystis Lutkeana} are
         the great kelps of the Pacific Ocean.

   {Kelp crab} (Zo["o]l.), a California spider crab ({Epialtus
      productus}), found among seaweeds, which it resembles in
      color.

   {Kelp salmon} (Zo["o]l.), a serranoid food fish ({Serranus
      clathratus}) of California. See {Cabrilla}.

Source : WordNet®

kelp
     n : large brown seaweeds having fluted leathery fronds