Sumerian abzu, Akkadian apsû — the body of fresh water understood to lie beneath the ground, filling the space between the earth's underside and the realm of the dead below it. It is the water that comes up in a well, stands in a marsh, and feeds a spring, and Babylonian cosmic geography treats it as a distinct layer of the universe with its own occupant.
The word's own etymology is not secure, which is worth saying because a confident derivation circulates: ab has been connected to the sea or to an opening and zu to knowing or to distance, and none of the combinations is agreed. What is certain is that the term is Sumerian and was borrowed into Akkadian, not the other way round.
A character who lasts three tablets
Enuma Elish personifies him, and the personification is short-lived by design. In the opening lines, before heaven or earth has been named, Apsu the fresh water and Tiamat the salt sea mingle their waters as a single body, and gods are generated inside them. The young gods are boisterous. Apsu cannot rest, and goes to Tiamat with a proposal to destroy them so he can sleep.
She refuses, and is talked round by his vizier Mummu — a figure of uncertain meaning whose name may relate to a word for craftsmanship or form, and who is one of the genuine puzzles of the poem. The plot leaks. Enki recites a spell that puts Apsu into a deep sleep, strips off his crown and radiance and puts them on himself, kills him, and ties up Mummu with a nose-rope.
Then comes the detail that makes the whole episode more than a fight. Enki builds his own house on top of the corpse, names it the Apsu, and lives in it — and it is inside that building that Marduk is conceived and born. The god of wisdom's residence is a defeated enemy converted into architecture, and the next generation of divine power is gestated inside it.
The temple courtyard
This is not only cosmology; it is a description of a building. Mesopotamian temples kept a water installation called an apsû — a basin or tank in the courtyard, holding water used for purification, sometimes substantial enough to be a fixed stone or brick construction. At Eridu, Enki's own city, the E-abzu stood where the water table is close to the surface, and the theology and the hydrology are the same fact.
That basin gives the exorcists their material. Purifying water in incantations is described as water of the Apsu, drawn from the pure deep, and the exorcist's authority is repeatedly traced to Enki's house. So the primordial being killed on the first tablet of the creation epic is also a fixture in the yard of every major sanctuary, filled each morning. Very few mythologies keep their defeated primordials on the premises.
Layers of the world
The Babylonian universe as reconstructed from the scholarly texts is stacked: heavens above, in more than one level; the earth's surface where people live; the Apsu beneath it; and the land of no return below that. First-millennium learned tablets describe the levels with their materials and their occupants — different stones for different heavens, and gods assigned to each. The Apsu's position in that stack matters for how the tradition thinks about depth: going down does not immediately mean death. There is a layer of fresh water, wisdom and craft in between, which is why Enki is a god of the below without being a god of the dead.
Points worth clearing up
Does the English word "abyss" come from it?
No, though the resemblance is striking and the claim is repeated constantly. Greek abyssos is a native Greek formation — the negative prefix a- plus byssos, depth or bottom — meaning "bottomless", and it enters Latin and then English through the Greek Bible. There is no evidence of borrowing from Akkadian or Sumerian, and the sound similarity is coincidence, of the same order as the Ishtar-and-Easter claim taken apart on the Inanna page. What is true is that the biblical Hebrew tehom, the deep of Genesis, is genuinely cognate with Akkadian tiāmtu — a real linguistic relationship involving the other half of the primordial pair, which makes the false one about Apsu especially easy to slip into.
Why does he lose to a spell rather than a weapon?
Because the poem is establishing what kind of god Enki is before it establishes what kind Marduk is. Apsu is put to sleep by an incantation and killed while unconscious; Tiamat is fought in open battle with winds, net and arrow by a champion who has negotiated a contract first. Two victories, two methods, two theologies of power — the cunning technician and the licensed sovereign — and the poem's whole argument depends on the reader noticing that the second requires the first to have failed to be enough. It also flags something structural: nowhere in this literature is intelligence treated as a lesser sort of strength.
Is he a "god" at all, or just water?
Deliberately both, and the ambiguity is not sloppiness. The opening lines of the poem describe a state before names existed, and a Mesopotamian scribe understood that a thing without a name is not yet a distinct thing — the text says as much. Apsu is water that becomes a person for as long as the story requires a person and then reverts to being a place where a god lives and a basin in a courtyard. The same is true of his consort. Reading these two as characters with psychology, on the model of later mythologies, imports assumptions the Babylonian material does not carry, and the tradition overview explains why that caution applies right across the corpus.