Something that is very large and powerful.
From the Hebrew (via Middle English from Late Latin) for twisted or coiled, ‘leviathan’ is used in the Bible to refer to a serpent or dragon:
Canst thou draw out leviathan with an hook? or his tongue with a cord which thou lettest down? (Job 41:1)
Thou brakest the heads of leviathan in pieces, and gavest him to be meat to the people inhabiting the wilderness. (Psa 74:14)
There go the ships: there is that leviathan, whom thou hast made to play therein. (Psa 104:26)
In 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. (Isa 27:1)
According to a midrash, the leviathan was created on the fifth day (Yalḳ., Gen. 12). Originally God produced a male and a female leviathan, but lest in multiplying the species should destroy the world, He slew the female, reserving her flesh for the banquet that will be given to the righteous on the advent of the Messiah.
His abode is the Mediterranean Sea; and the waters of the Jordan fall into his mouth (Bek. 55b; B. B. l.c.);
in spite of his supernatural strength, the leviathan is afraid of a small worm called ‘kilbit’, which clings to the gills of large fishes and kills them (Shab. 77b).
Yet in everyday expression, the word has become synonymous with any large sea monster or creature — essentially, with a whale.
Merriam-Webster details that Leviathan refers specifically to “a sea monster defeated by Yahweh in various scriptural accounts”. Since Thomas Hobbes’ 1651 political treatise, it has also come to refer to a political state, especially a totalitarian state having a vast bureaucracy.
As common noun, leviathan is broadly used to refer to anything that is large and formidable.
There are great parallels here with the Book of Revelation, which describes two beasts: the first of which comes “out of the sea” and is given authority and power by the dragon; and the second comes “out of the earth” (see Behemoth).
Both leviathan and behemoth are prominent in Jewish eschatology. Behemoth will be prominent in next week’s nom du jour.