This word has become synonymous with any large sea monster or creature. In literature (e.g., Herman Melville's Moby-Dick) it refers to great whales, and in Modern Hebrew, it simply means "whale". It is described extensively in Job 3:8, Job 40:15-41:26, Amos 9:3, Psalms 74:13–23, Psalm 104:26 and Isaiah 27:1.
The Leviathan is mentioned six times in the Tanakh, with Job 41:1–34 being dedicated to describing him in detail: "Behold, the hope of him is in vain; shall not one be cast down even at the sight of him?" In Psalms 74, God is said to "break the heads of Leviathan in pieces" before giving his flesh to the people of the wilderness. In Psalms 104, God is praised for having made all things, including Leviathan, and in Isaiah 27:1, he is called the "wriggling serpent" who will be killed at the end of time.
Ancient Near Eastern origins
The Hebrew Leviathan was a development of the earlier Canaanite sea monster Lôtān or Litanu (Ugaritic: Ltn) described as a servant of the sea god Yammu in the Baal Cycle discovered in the ruins of Ugarit. The account has gaps, making it unclear whether some phrases describe him or other monsters at Yammu's disposal such as Tunannu (the Biblical Tannin). Most scholars agree on describing Lôtān as "the fugitive serpent" (bṯn brḥ) but he may or may not be "the wriggling serpent" (bṯn ʿqltn) or "the mighty one with seven heads" (šlyṭ d.šbʿt rašm). Like Yammu's other servants and Yammu himself, Lôtān is defeated by the benevolent storm god Baʿal. His role seems to have been prefigured by the earlier serpent Têmtum whose death at the hands of the benevolent storm god Hadad is depicted in Syrian seals of the 18th–16th century BCE.
Sea serpents feature prominently in the mythology of the Ancient Near East. They are attested by the 3rd millennium BCE in Sumerian iconography depicting the god Ninurta overcoming a seven-headed serpent. It was common for Near Eastern religions to include a Chaoskampf: a cosmic battle between a sea monster representing the forces of chaos and a creator god or culture hero who imposes order by force. The Babylonian creation myth describes Marduk's defeat of the serpent goddess Tiamat, whose body was used to create the heavens and the earth. As early as 1894, scholars began to note the similarity of these earlier stories and the references to Leviathan's battle with Yahweh found in the Hebrew Scriptures. The mention of the Tannins in the Genesis creation narrative (translated as "great whales" in the King James Version) and Leviathan in the Psalms do not describe them as harmful but as ocean creatures who are part of God's creation. The element of competition between God and the sea monster and the use of Leviathan to describe the powerful enemies of Israel may reflect the influence of the Mesopotamian and Canaanite legends or the contest in Egyptian mythology between the Apep snake and the sun god Horus. Alternatively, the removal of such competition may have reflected an attempt to naturalize Leviathan in a process that demoted it from deity to demon to monster.
In later Jewish literature
When the Jewish midrash (explanations of the Tanakh) were being composed, it was held that God originally 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 (B. B. 74b).
the...sea monsters: The great fish in the sea, and in the words of the Aggadah (B.B. 74b), this refers to the Leviathan and its mate, for He created them male and female, and He slew the female and salted her away for the righteous in the future, for if they would propagate, the world could not exist because of them. הַתַּנִינִם is written. [I.e., the final “yud,” which denotes the plural, is missing, hence the implication that the Leviathan did not remain two, but that its number was reduced to one.] – [from Gen. Rabbah 7:4, Midrash Chaseroth V’Yetheroth, Batei Midrashoth, vol 2, p. 225].
In the Talmud Baba Bathra 75a it is told that the Leviathan will be slain and its flesh served as a feast to the righteous in [the] Time to Come, and its skin used to cover the tent where the banquet will take place. The festival of Sukkot (Festival of Booths) therefore concludes with a prayer recited upon leaving the sukkah (booth): "May it be your will, Lord our God and God of our forefathers, that just as I have fulfilled and dwelt in this sukkah, so may I merit in the coming year to dwell in the sukkah of the skin of Leviathan. Next year in Jerusalem."
The enormous size of the Leviathan is described by Johanan bar Nappaha, from whom proceeded nearly all the aggadot concerning this monster: "Once we went in a ship and saw a fish which put his head out of the water. He had horns upon which was written: 'I am one of the meanest creatures that inhabit the sea. I am three hundred miles in length, and enter this day into the jaws of the Leviathan'" (B. B. l.c.).
When the Leviathan is hungry, reports Rabbi Dimi in the name of Rabbi Johanan, he sends forth from his mouth a heat so great as to make all the waters of the deep boil, and if he would put his head into Paradise no living creature could endure the odor of him (ib.). His abode is the Mediterranean Sea; and the waters of the Jordan fall into his mouth (Bek. 55b; B. B. l.c.).
In a legend recorded in the Midrash called Pirke de-Rabbi Eliezer it is stated that the fish which swallowed Jonah narrowly avoided being eaten by the Leviathan, which eats one whale each day.
The body of the Leviathan, especially his eyes, possesses great illuminating power. This was the opinion of Rabbi Eliezer, who, in the course of a voyage in company with Rabbi Joshua, explained to the latter, when frightened by the sudden appearance of a brilliant light, that it probably proceeded from the eyes of the Leviathan. He referred his companion to the words of Job xli. 18: "By his neesings a light doth shine, and his eyes are like the eyelids of the morning" (B. B. l.c.). However, in spite of his supernatural strength, the leviathan is afraid of a small worm called "kilbit", which clings to the gills of large fish and kills them (Shab. 77b).
In the eleventh century piyyut (religious poem), Akdamut, recited on Shavuot (Pentecost), it is envisioned that, ultimately, God will slaughter the Leviathan, which is described as having "mighty fins" (and, therefore, a kosher fish, not an inedible snake or crocodile), and it will be served as a sumptuous banquet for all the righteous in Heaven.
The Leviathan of the Middle Ages was used as an image of Satan, endangering both God's creatures—by attempting to eat them—and God's creation—by threatening it with upheaval in the waters of Chaos. St. Thomas Aquinas described Leviathan as the demon of envy, first in punishing the corresponding sinners (Secunda Secundae Question 36). Peter Binsfeld likewise classified Leviathan as the demon of envy, as one of the seven Princes of Hell corresponding to the seven deadly sins. Leviathan became associated with, and may originally have referred to, the visual motif of the Hellmouth, a monstrous animal into whose mouth the damned disappear at the Last Judgement, found in Anglo-Saxon art from about 800, and later all over Europe.
Young Earth Creationists argue that both the scale-clad, fire-breathing Leviathan, and the Behemoth depicted in the Bible could only have been dinosaurs that they allege still existed when the Book of Job was written.
The Revised Standard Version of the Bible  suggests in a footnote to Job 41:1 that Leviathan may be a name for the crocodile, and in a footnote to Job 40:15, that Behemoth may be a name for the hippopotamus.
In LaVeyan Satanism, according to the author of The Satanic Bible, Anton Szandor LaVey, Leviathan represents the element of Water and the direction of west. The element of water in Satanism is associated with life and creation, and may be represented by a Chalice during ritual. In The Satanic Bible, Leviathan is listed as one of the Four Crown Princes of Hell. This association was inspired by the demonic hierarchy from The Book of the Sacred Magic of Abra-Melin the Mage. The Church of Satan uses the Hebrew letters at each of the points of the Sigil of Baphomet to represent Leviathan. Starting from the lowest point of the pentagram, and reading counter-clockwise, the word reads "לִוְיָתָן". Transliterated, this is (LVIThN) Leviathan. In demonology, the Leviathan is one of the seven princes of Hell (envy) and its gatekeeper (see Hellmouth).
In novels and most of the Hollywood movies the leviathan is portrayed as a humongous sea animal, capable of devastating entire fleets. In Pirates of the Caribbean: At World's End the leviathan is represented as kraken. In literature, lord Tennyson wrote a sonnet the kraken which describes the massive creature that dwell at the bottom of the sea 
Below the thunders of the upper deep;
Far far beneath in the abysmal sea,
His ancient, dreamless, uninvaded sleep
The Kraken sleepeth: faintest sunlights flee
About his shadowy sides; above him swell
Huge sponges of millennial growth and height;
And far away into the sickly light,
From many a wondrous grot and secret cell
Unnumber'd and enormous polypi
Winnow with giant arms the slumbering green.
There hath he lain for ages, and will lie
Battening upon huge seaworms in his sleep,
Until the latter fire shall heat the deep;
Then once by man and angels to be seen,
In roaring he shall rise and on the surface die.
- Cirlot, Juan Eduardo (1971). A Dictionary of Symbols (2nd ed.). Dorset Press. p. 186.
- K. van der Toorn, Bob Becking, Pieter Willem van der Horst, eds. (1999). Dictionary of deities and demons in the Bible. Wm. B. Eerdmans Publishing. pp. 512–514. Retrieved 13 July 2012.
- Uehlinger (1999), p. 514.
- Herrmann (1999), p. 133.
- Heider (1999).
- Uehlinger (1999), p. 512.
- Hermann Gunkel, Heinrich Zimmern; K. William Whitney Jr., trans., Creation And Chaos in the Primeval Era And the Eschaton: A Religio-historical Study of Genesis 1 and Revelation 12. (Grand Rapids: MI: Erdmans, 1895, 1921, 2006).
- Enuma Elish, Tablet IV, lines 104–105, 137–138, 144 from Heidel, Babylonian Genesis, 41–42.
- Gen. 1:21.
- Gen. 1:21 (KJV).
- Ps. 104.
- For example, in Isaiah 27:1.
- Watson, R.S. (2005). Chaos Uncreated: A Reassessment of the Theme of "chaos" in the Hebrew Bible. Walter de Gruyter. ISBN 3110179938, 9783110179934
- Chabad. "Rashi's Commentary on Genesis". Retrieved 25 October 2012.
- Finkel, Avraham (1993). The Essence of the Holy Days: Insights from the Jewish Sages. Northvale, N.J.: J. Aronson. p. 99. ISBN 0-87668-524-6. OCLC 27935834.
- Hirsch, Emil G.; Kaufmann Kohler; Solomon Schechter; Isaac Broydé. "Leviathan and Behemoth". JewishEncyclopedia.com. Retrieved 3 September 2009.
- Labriola, Albert C. (1982). "The Medieval View of History in Paradise Lost". In Mulryan, John. Milton and the Middle Ages. Bucknell University Press. pp. 115–34. ISBN 978-0-8387-5036-0. p. 127.
- Link, Luther (1995). The Devil: A Mask Without a Face. London: Reaktion Books. pp. 75–6. ISBN 0-948462-67-1.
- Hofmann, Petra (2008). Infernal Imagery in Anglo-Saxon Charters (PDF) (Thesis). St Andrews. pp. 143–4.
- "Genesis Park, Room 1: The Dinosaurs". Genesispark.com. Retrieved 13 July 2012.
- Taylor, Paul S. (13 February 2008). "Were Dinosaurs alive after Babel?". Answersingenesis.org. Retrieved 13 July 2012.
- The Holy Bible Revised Standared Version. New York: Thomas Nelson and Sons. 1959. pp. 555–556
- "The History of the Origin of the Sigil of Baphomet and its Use in the Church of Satan". Church of Satan website. Retrieved 3 September 2009.
- This article incorporates text from a publication now in the public domain: Singer, Isidore; et al., eds. (1901–1906). "leviathan". Jewish Encyclopedia. New York: Funk & Wagnalls Company.
- Heider, George C. (1999), "Tannîn", Dictionary of Deities and Demons in the Bible, 2nd ed., Grand Rapids: Wm. B. Eerdmans Publishing, pp. 834–836.
- Herrmann, Wolfgang (1999), "Baal", Dictionary of Deities and Demons in the Bible, 2nd ed., Grand Rapids: Wm. B. Eerdmans Publishing, pp. 132–139.
- Uehlinger, C. (1999), "Leviathan", Dictionary of Deities and Demons in the Bible, 2nd ed., Grand Rapids: Wm. B. Eerdmans Publishing, pp. 511–515.
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- 'Sea monster' whale fossil unearthed. Named Leviathan by scientists 30 June 2010.
- Putting God on Trial – The Biblical Book of Job contains a major section on the literary use of Leviathan.
- http://www.biblegateway.com/passage/?search=job%2041:1-41:34&version=KJV Job|41:1–41:34|KJV
- The fossilised skull of a colossal "sea monster" has been unearthed along the UK's Jurassic Coast. 27 October 2009
- 'Sea monster' whale fossil unearthed 30 June 2010
- Enuma Elish (Babylonian creation epic)
- Philologos concordance page
- Text of the Leviathan passage from Job 40 and 41