(in biblical use) a sea monster, identified in different passages with the whale and the crocodile (e.g., Job 41, Ps. 74:14), and with the Devil (after Isa. 27:1). a very large aquatic creature, especially a whale:the great leviathans of the deep a thing that is very large or powerful, especially a ship. an autocratic monarch or state.
[with allusion to Hobbes' Leviathan (1651)]