Concise Bible Dictionary:

This is really a Hebrew word (livyathan), and is generally believed to refer to any great sea or land monster, and especially to the crocodile, which comparatively lately has been met with in the river Kishon in Palestine. The minute description given in Job 41 agrees with what is known of the crocodile. He cannot be taken with a hook, nor his flesh be filled with barbed irons: his scales, which are very close, protect him. Fire proceeding from his mouth is figurative language. The whole account is given to contrast the mighty power of God in His works, with the littleness of Job.
The crocodile is able to remain a long time under water without breathing, and can thus approach a waterfowl unperceived and hold it down till it is drowned. It then tears it to pieces with its teeth, and swallows the pieces whole.It is thus an apt symbol of the enemy of God’s people (Psalm 74:1414Thou brakest the heads of leviathan in pieces, and gavest him to be meat to the people inhabiting the wilderness. (Psalm 74:14)).
In Isaiah 27:11In that day the Lord with his sore and great and strong sword shall punish leviathan the piercing serpent, even leviathan that crooked serpent; and he shall slay the dragon that is in the sea. (Isaiah 27:1) it also typifies Satan: “leviathan the piercing serpent, even leviathan that crooked serpent,” whom God will punish. In Psalm 104:2626There go the ships: there is that leviathan, whom thou hast made to play therein. (Psalm 104:26) the reference may be to any sea monster, for it is in connection with the “great and wide sea,” that is, the Mediterranean. In Job 3:88Let them curse it that curse the day, who are ready to raise up their mourning. (Job 3:8) it should be translated “leviathan,” instead of “their mourning,” and this confirms the general meaning of some monster.

Jackson’s Dictionary of Scripture Proper Names:

a coiled animal: their burrowing: their union