Short answers, pulled from the story.
Tiamat is the cosmic ocean in ancient Mesopotamian religion, a primordial deity who mates with Abzu, the groundwater, to produce the Babylonian pantheon. She is described as both the mother of the gods and, after Abzu's death, a war-bringer who fashions eleven monsters from her own body. Her story is recorded in the Enuma Elis, a creation epic written in the 2nd millennium BCE.
Tiamat's name derives from the Akkadian word for sea, tamtu, following the early form tiamtum. Scholars Thorkild Jacobsen and Walter Burkert both argued for this connection. The name became so interchangeable with the ordinary word for sea that some Akkadian copyists simply substituted tamtu for Tiamat in copies of the original texts.
Marduk, the storm-god and son of Enki, sliced Tiamat in half. From her ribs he made the vault of heaven and earth, her weeping eyes became the sources of the Tigris and the Euphrates, and her tail became the Milky Way. He then took the Tablet of Destinies from her consort Qingu and installed himself as head of the Babylonian pantheon.
Tiamat fashioned eleven monsters from her own body to battle the gods: Basmu the Venomous Snake, Usumgallu the Great Dragon, Musmahhu the Exalted Serpent, Mushussu the Furious Snake, Lahmu the Hairy One, Ugallu the Big Weather-Beast, Uridimmu the Mad Hound, Girtablullu the Scorpion Man, Umu dabrutu the Violent Storms, Kulullu the Fish-Man, and Kusarikku the Bull-Man. The epic describes their bodies as filled with poison instead of blood.
A connection has been claimed between Tiamat and the Northwest Semitic word tehom, which appears in Genesis 1:2 as the word for the deep. The linguistic relationship suggests a shared ancient cultural memory of a primordial watery chaos.
The depiction of Tiamat as a multi-headed dragon was popularized in the 1970s through Dungeons and Dragons, the role-playing game, which drew on earlier sources linking Tiamat to figures such as Lotan, also known as Leviathan. Assyriologist Alexander Heidel had previously noted that a dragon form cannot be attributed to Tiamat with certainty based on the ancient sources alone.