The Collaborative International Dictionary of English v.0.48:

Leviathan \Le*vi"a*than\ (l[-e]*v[imac]"[.a]*than), n. [Heb.
   livy[=a]th[=a]n.]
   [1913 Webster]
   1. An aquatic animal, described in the book of Job, ch. xli.,
      and mentioned in other passages of Scripture.
      [1913 Webster]

   Note: It is not certainly known what animal is intended,
         whether the crocodile, the whale, or some sort of
         serpent.
         [1913 Webster]

   2. The whale, or a great whale. --Milton.
      [1913 Webster]

WordNet (r) 3.0 (2006):

leviathan
    n 1: the largest or most massive thing of its kind; "it was a
         leviathan among redwoods"; "they were assigned the
         leviathan of textbooks"
    2: monstrous sea creature symbolizing evil in the Old Testament

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "leviathan":
   argosy, bark, boat, bottom, bucket, craft, cyclopean, dinosaur,
   elephant, elephantine, enormous, gargantuan, giant, gigantic,
   hippo, hippopotamus, hooker, hulk, hull, immense, jumbo, keel,
   mammoth, mastodon, monster, packet, ship, tub, vessel, watercraft,
   whale



Easton's 1897 Bible Dictionary:

Leviathan
   a transliterated Hebrew word (livyathan), meaning "twisted,"
   "coiled." In Job 3:8, Revised Version, and marg. of Authorized
   Version, it denotes the dragon which, according to Eastern
   tradition, is an enemy of light; in 41:1 the crocodile is meant;
   in Ps. 104:26 it "denotes any large animal that moves by
   writhing or wriggling the body, the whale, the monsters of the
   deep." This word is also used figuratively for a cruel enemy, as
   some think "the Egyptian host, crushed by the divine power, and
   cast on the shores of the Red Sea" (Ps. 74:14). As used in Isa.
   27:1, "leviathan the piercing [R.V. 'swift'] serpent, even
   leviathan that crooked [R.V. marg. 'winding'] serpent," the word
   may probably denote the two empires, the Assyrian and the
   Babylonian.


The Devil's Dictionary (1881-1906):

LEVIATHAN, n.  An enormous aquatic animal mentioned by Job.  Some
suppose it to have been the whale, but that distinguished
ichthyologer, Dr. Jordan, of Stanford University, maintains with
considerable heat that it was a species of gigantic Tadpole (_Thaddeus
Polandensis_) or Polliwig -- _Maria pseudo-hirsuta_.  For an
exhaustive description and history of the Tadpole consult the famous
monograph of Jane Potter, _Thaddeus of Warsaw_.