The first oracle at Delphi was commonly known as Sybil, though her name was Herophile. She sang her predictions, which she received from Gaia. The word Sybil is derived from the more ancient Greek term Kybele, and there were many Sybils in the ancient world.
Later, Sibyl or Pythia became a title given to whichever priestess manned the oracle at the time. The Sibyl sat on the Sybilline Rock, breathing in vapors from the ground and eating laurel leaves, gaining her often puzzling predictions from that. Pausanias claimed that the Sybil 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 Sybil received her powers from Gaia originally, who passed the oracle to Thetis, who passed it to Phoebe.
The term can also apply to prophetesses from other oracles. The Cumaean Sybil and the Erythraean Sybil are especially notable.
Sybil is also the name of a 1973 book and 1976 movie about multiple personality disorder.