In its modern usage, the word "oracle" is used to describe either a prophet inspired by spiritual forces, or to describe a particular prophesy. In ancient Greece, the oracle was a place where these divinely-inspired prophesies of the future were passed down to mortals. Usually these prophesies were given in response to questions, but sometimes they flowed out randomly from the priest or priestess acting as an intermediary. To be an oracle, the place needed to have a variable and periodic attribute that could be subject to the interpretation of the priesthood. The priests would then ascribe both the event and the interpretation of the event to their patron god or goddess. For example, the ancient oracles of Zeus were areas where priests could interpret the wind rustling through the trees.

An exception to this definition was the oracle of Asclepius at Epidaurus, where the sick were treated with something akin to faith-healing and hypnosis. Although the priests performed the healings with no local natural events to inspire them, these miracles were attributed to the place and the divine powers that resided there.

The most famous oracle was that of Apollo at Delphi, discovered as a fissure in the side of Mt. Parnassus emitting a gas that would cause seizures among the goats that grazed nearby. The convulsions and wild ravings of a goatherd who was also affected were interpreted by the locals as "divine inspiration", and the priesthood moved in rapidly to take advantage of the unusual situation. The oracle was ascribed to a few other deities before the temple of Apollo was established. The Pythia was the priestess of this oracle who was crowned in laurel and seated on a tripod perched over the cleft that produced the intoxicating vapors. Her utterances while under the influence were usually so disjointed that additional clergy were needed to provide interpretation.