What was Poseidon the goddess of?

Poseidon was best-known as the Greek god of the sea. He was also the god of earthquakes and the god of horses. So, if you are asking his domain with respect to where he lived, that would be in the ocean or on the ocean floor, although the primary gods also had residences on Olympus. If you are asking his domain with respect to what he ruled over as a god, that would be the sea, earthquakes, and horses.