Source : Webster's Revised Unabridged Dictionary (1913)

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

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

   2. The whale, or a great whale. --Milton.

Source : WordNet®

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