Tiamat is a personification of the primordial sea from which the gods were first created. She is also the main adversary of Marduk in the Enūma Eliš.
Tiamat's exact functions as a goddess are difficult to establish. As her name indicates (see below), she was a deification of the primordial sea. Our best source of information for Tiamat is the myth Enūma Eliš, and in fact, there are only a handful of references to her outside of it. Enūma Eliš begins with a description of the two primeval seas, the salt sea Tiamat and the sweet sea Abzu, mingling their waters together to create the gods (for recent translations of the story see Foster 2005: 436-486 and Lambert 2013). In the following battle between Abzu and Ea, Tiamat attempts to appease Abzu and stop the conflict. But when she is later pressured by the lower gods to revenge him, she herself becomes the main antagonist of the story, creating an army of monsters led by her new consort, Qingu. She is ultimately defeated by Marduk, who incapacitates her with his "Evil Wind" and then kills her with an arrow. Marduk splits her in two, creating heaven and earth from her body, the Tigris and Euphrates from her eyes, mist from her spittle, mountains from her breasts and so on. Throughout the epic, there are differing descriptions of Tiamat: she appears both as a body of water, as a human figure, and as having a tail (Tablet V, line 59). These varying descriptions are ultimately reconciled as Marduk turns her limbs into geographical features.
In Enūma Eliš, Tiamat is the mother of all the gods (Tablet I, line 4). Together with Abzu she creates Lahmu and Lahamu, who in turn beget Anšar and Kišar.
Though one cannot point to a syncretism as such, there are several models for Tiamat in the earlier mythology. Katz (2011: 18f) argues that the figure of Tiamat unites two strands of tradition attached to the sea. The first is the motherly figure of Namma, who is also referred to as a primeval ocean from which the gods were created. The other is the figure of the sea as a monstrous adversary, like the Levantine god Yamm (see also Jacobsen 1968: 107). Another important influence for the figure of Tiamat is Anzu, a mythical bird defeated by Ninurta, indeed the battle between Marduk and Tiamat has a number of parallels to the battle between Ninurta and Anzu (Lambert 1986).
There was no cult dedicated directly to Tiamat, but the battle between Tiamat and Marduk played an important role in the New Year's festival in Babylon. The Enūma Eliš was recited on its fourth day, and some argue that the festival included a symbolic reenactment of the mythological battle (see the discussion in Lambert 2013: 461f).
The oldest attestation of Tiamat is an Old Akkadian incantation (Westenholz 1974: 102), though there are few other references to her until the first millenium BCE (see Lambert 2013: 237). After the composition of the Enūma Eliš, Tiamat is found in a number of theological commentary works, but most of these seem to rely on the epic (e.g. SAA 3.39, r. 1-3). Tiamat is also mentioned by Berossus, writing in the 3rd century BCE (Breucker 2011: 648f).
A relief from the temple of Bêl in Palmyra depicts Nabu and Marduk slaying Tiamat, who is shown with a woman's body and legs made of snakes (Dirven 1997). However, this scene is a late Hellenistic adoption of the Babylonian motif, and no Mesopotamian image has been positively identified as a representation of Tiamat. A string of identifications (Yadin 1971, Grafman 1972, Kaplan_1976) have recently been rejected (George_2012), and until new evidence surfaces they remain dubious.
The name Tiamat is uncontracted form of the word tâmtu, meaning "sea". The long vowel â is contracted from the short vowels i and a. The word is in the "absolute state," a noun form that is equivalent to the vocative (a grammatical case which directly invokes or addresses a person or deity; literally the name means "O, sea!").
Written forms: dti-amat, ti-amat, dtam-tum, ti-àm-tim, ta-à-wa-ti
Normalised forms: Tiamat, Tiāmat, Tiʼamat, Thalatta (Greek).
Sophus Helle, 'Tiamat (goddess)', Ancient Mesopotamian Gods and Goddesses, Oracc and the UK Higher Education Academy, 2016 [http://oracc.museum.upenn.edu/amgg/listofdeities/tiamat/]