The Collaborative International Dictionary of English v.0.48:

Sea otter \Sea" ot"ter\ (Zool.)
   An aquatic carnivore (Enhydris lutris syn. Enhydris
   marina) found in the North Pacific Ocean. Its fur is highly
   valued, especially by the Chinese. It is allied to the common
   otter, but is larger, with feet more decidedly webbed.
   [1913 Webster]

   Sea-otter's cabbage (Bot.), a gigantic kelp of the Pacific
      Ocean (Nereocystis Lutkeana). See Nereocystis.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Otter \Ot"ter\ ([o^]t"t[~e]r), n. [OE. oter, AS. otor; akin to
   D. & G. otter, Icel. otr, Dan. odder, Sw. utter, Lith. udra,
   Russ, vuidra, Gr. "y`dra water serpent, hydra, Skr. udra
   otter, and also to E. water. [root]137, 215. See Water, and
   cf. Hydra.]
   [1913 Webster]
   1. (Zool.) Any carnivorous animal of the genera Lutra,
      Enhydra, and related genera of the family Mustelidae.
      Several species are described. They have large, flattish
      heads, short ears, and webbed toes. They are aquatic, and
      feed on fish. The sea otter (Enhydra lutris) also eats
      clams, crabs, starfish, abalone, and other marine animals;
      they may come to the surface, and lying on their backs
      using the stomach as a table, may be seen cracking open
      the shell of its prey with a rock. The common otter of
      Europe is Lutra vulgaris; the North American otter (or
      American otter) is Lutra Canadensis, which inhabits
      marshes, streams and rivers; other species inhabit South
      America and Asia. The North American otter adult is about
      three to four feet long (including the tail) and weighs
      from 10 to 30 pounds; the sea otter is commonly four feet
      long and 45 pounds (female) or 60 pounds (male). Their fur
      is soft and valuable, and in the nineteenth century they
      were hunted extensively. The sea otter was hunted to near
      extinction by 1900, and is now protected. Fewer than 3,000
      sea otters are believed to live along the central
      California coast.
      [1913 Webster +PJC]

   2. (Zool.) The larva of the ghost moth. It is very injurious
      to hop vines.
      [1913 Webster]

   Otter hound, Otter dog (Zool.), a small breed of hounds,
      used in England for hunting otters; see otterhound .

   Otter sheep. See Ancon sheep, under Ancon.

   Otter shell (Zool.), very large bivalve mollusk
      (Schizothaerus Nuttallii) found on the northwest coast
      of America. It is excellent food, and is extensively used
      by the Indians.

   Sea otter. (Zool.) See in the Vocabulary.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sea otter
    n 1: large marine otter of northern Pacific coasts having very
         thick dark brown fur [syn: sea otter, Enhydra lutris]