Tiamat (Akkadian Tiāmat) is the Babylonian primordial goddess of the saltwater ocean — the personification of the chaotic primeval waters that exist before the cosmos is ordered. She is the antagonist of the Babylonian creation epic Enuma Elish, where the young storm-god Marduk kills her and forms the heavens and the earth from her divided body. She is also the etymological sister of the Hebrew tehom — "the deep" — in Genesis 1:2.
Akkadian Tiāmat and Hebrew tehom (תְּהוֹ×) descend from a common Proto-Semitic root, *t-h-m, meaning the primordial sea or abyss. Tiamat is the personalized goddess. Tehom is the impersonal abyss. Same word, different theological treatment.
The cognate relationship is one of the clearest pieces of evidence that Genesis 1 is participating in — and revising — a much older Mesopotamian creation pattern. The Hebrew authors didn't write in a vacuum. They wrote inside a shared regional vocabulary about how the world begins, and the choices they made inside that vocabulary are the theology.
The Enuma Elish ("When on high…") is the Babylonian creation epic, preserved on seven cuneiform tablets and recited at the New Year festival in Babylon. Its plot, in brief:
At the beginning, only two beings exist: Apsu, the freshwater abyss (male), and Tiamat, the saltwater ocean (female). Their commingling produces the first generation of gods. The younger gods are noisy and disruptive, and Apsu plots to destroy them. Ea, the god of wisdom, kills Apsu first.
Tiamat rises in revenge. She creates an army of monsters and takes a new consort, Kingu, to lead them. The younger gods are terrified. Marduk, the young storm-god, volunteers to fight her — but only on the condition that he be made king of all the gods. They agree.
Marduk defeats Tiamat in single combat. He drives the four winds into her mouth so she cannot close it, then kills her with arrows. He splits her body in half, the text says, "like a flat fish." From one half he makes the heavens; from the other, the earth. He fixes the stars, sets the seasons, and establishes the order of the cosmos. Order, in this story, is wrested from the body of chaos.
Tiamat is variously imagined as a sea-dragon, a serpent, a primordial monster. She is the personification of watery chaos that must be subdued for order to exist. She is the original of a type that recurs across the ancient Near East: Yam (the Canaanite sea-god), Lotan or Leviathan (the sea-serpent of Ugaritic and Hebrew tradition), and Rahab (the chaos-monster of Job and Isaiah). The combat-with-the-chaos-sea pattern is so widespread that scholars have a name for it — Chaoskampf.
Tehom in Genesis 1:2 — "darkness was over the face of the deep" — is the demythologized Tiamat. The Hebrew authors knew the pattern and stripped the personhood from it. No goddess to slay. No army of monsters. No single combat. Just passive chaotic waters that Elohim orders by speech.
The demythologization is the theology. Israel's God does not have to fight chaos for the cosmos to exist. There is no rival. Compare the Enuma Elish and Genesis 1 directly and the polemic shows: the same primordial setup, a radically different cosmology — order by command rather than order by conquest.
In modern pop culture Tiamat survives mostly as a stock fantasy dragon — Dungeons & Dragons gave her five heads and made her the queen of evil chromatic dragons, and most contemporary readers who know the name know it from there. That use has wildly little to do with her actual role in the mythological record. The fantasy-dragon Tiamat is a few decades old. The cosmogonic mother-goddess of the chaotic deep is roughly three thousand years older than that, and is doing entirely different work.
The standard accessible English translation of the Enuma Elish is in Stephanie Dalley, Myths from Mesopotamia (Oxford World's Classics). The older scholarly translation is Alexander Heidel, The Babylonian Genesis (1951), still useful for its parallel-text presentation alongside Genesis. For the visual and ritual side — what these figures looked like and how they were used — Frans Wiggermann's Mesopotamian Protective Spirits and his shorter iconographic studies are the standard references.