Delphi is a city of ancient Greece, home to the Delphic Oracle, dedicated to the god Apollo and inhabited by the nymph Cassotis, which was revered throughout the ancient world. The ancient Greeks considered Delphi the centre of the universe.
Delphi was located in a plateau on the side of Mt. Parnassus. This semicircular spur is known as Phaedriades; it overlooks the Pleistos Valley. Southwest of Delphi, about 15 km away, is the harbor-city of Kirrha in the Corinthian Gulf.
The name "Delphi" probably stems from Delphinios, an epithet for Apollo stemming from his connection to dolphins. Apollo allegedly came to Delphi with Cretan priests on the backs of dolphins. According to another legend, Apollo walked to Delphi from the north and stopped at Tempe, a city in Thessaly to pick laurel, a plant sacred to him. In commemoration of this legend, the winners at the Pythian Games received a laurel wreath picked in Tempe.
As a young man, Apollo killed the vicious dragon Python, which lived in Delphi beside the Castalian Spring, according to some because Python had attempted to rape Leto while she was pregnant with Apollo and Artemis. This was the spring which emitted vapors that caused the Oracle at Delphi to give her prophesies. Apollo killed Python but had to be punished for it, since Python was a child of Gaia. The shrine dedicated to Apollo was probably originally dedicated to Gaia and then Poseidon. The oracle at that time predicted the future based on the lapping water and leaves rustling in the trees.
The first oracle at Delphi was commonly known as Sibyl, though her name was Herophile. She sang her predictions, which she received from Gaia. Later, "Sibyl" became a title given to whichever priestess manned the oracle at the time. The Sibyl sat on the Sibylline Rock, breathing in vapors from the ground and gaining her often puzzling predictions from that. Pausanias claimed that the Sibyl was "born between man and goddess, daughter of sea monsters and an immortal nymph". Others said she was sister or daughter to Apollo. Still others claimed the Sibyll received her powers from Gaia originally, who passed the oracle to Thetis, who passed it to Phoebe.
This oracle exerted considerable influence across the country, and was consulted before all major undertakings -- wars, the founding of colonies, and so forth. She also was respected by the semi-Hellenic countries around the Greek world, such as Macedonia, Lydia, Caria, and even Egypt. Croesus of Lydia consulted Delphi before attacking Persia, and according to Herodotus received the answer "if you do, you will destroy a great empire." Croesus found the response favorable and attacked, and was utterly overthrown (resulting, of course, in the destruction of his own empire).
The oracle is also said to have proclaimed Socrates the wisest man in Greece, to which Socrates said that if so, this was because he alone was aware of his own ignorance. In the 3rd century A.D., the oracle (perhaps bribed) declared that the god would no longer speak there.