The Leviathan appears in more than one religion. In Canaanite mythology and literature, it is a monster called Lotan, 'the fleeing serpent, the coiling serpent, the powerful with the seven heads'. It was eventually killed by Baal. The Leviathan is also the Ugaritic god of evil.
Literally, "coiled". In the Bible, and especially the Old Testament, the Leviathan is some sort of chaos animal in the shape of a crocodile or a serpent. In other bible texts it is taken to mean a whale or dolphin, because the animal is there described as living in the sea. Later the Leviathan became a symbol of evil, an anti-divine power (some sort of devil) which will be destroyed on Judgement Day.
"This great and wide sea, wherein are things creeping innumerable, both small and great beasts. There go the ships: there is that leviathan, whom thou hast made to play therein."
-- Ps. civ, 25-26