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

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_.