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.
      [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

kelp
    n 1: large brown seaweeds having fluted leathery fronds

Moby Thesaurus II by Grady Ward, 1.0:

54 Moby Thesaurus words for "kelp":
   algae, autophyte, bean, bracken, brown algae, climber, conferva,
   confervoid, creeper, diatom, fern, fruits and vegetables, fucus,
   fungus, grapevine, green algae, gulfweed, herb, heterophyte, ivy,
   legume, lentil, liana, lichen, liverwort, mold, moss, mushroom,
   parasite, parasitic plant, pea, perthophyte, phytoplankton,
   planktonic algae, plant families, puffball, pulse, red algae,
   rockweed, rust, saprophyte, sargasso, sargassum, sea lentil,
   sea moss, sea wrack, seaweed, smut, succulent, toadstool, vetch,
   vine, wort, wrack