Priestess of Delphi ~ the Oracle at Delphi, or the Delphic Oracle.

"The oracle of Delphi functioned in a specific place, the adyton, or "no entry" area of the temple's core, and through a specific person, the Pythia, who was chosen to speak, as a possessed medium, for Apollo, the god of prophecy."

"In the painting, "Priestess of Delphi" by The Honorable John Collier, a priestess - the Pythia - is depicted in a trance state, seated over a fissure in the rock through which vapors rise from the underground stream. In her left hand is a sprig of laurel - in Greek mythology, Apollo's sacred tree - and in the other hand a bowl meant to hold some of the water from the stream containing the gases."

Source

http://www.illusionsgallery.com/Story-Delphi.html

The Pythia was the priestess at Apollo's oracle in Delphi.

"The Pythia was the priestess at Apollo's oracle in Delphi. The name comes from Python, the dragon that was slain by Apollo. The Pythia operated as a vehicle for Apollo's will to be known to those on earth. A believer would make a sacrifice and present a question to a male priest. The male priest would then present the question to the Pythia.

The Pythia sat on a bronze tripod in the adytum, or inner chamber of Apollo's temple. In this sacred chamber the spirit of Apollo overcame the Pythia and inspired the prophecy. Some mythic traditions say the Pythia's trance was induced by vapors from a chasm below the temple or from chewing laurel leaves.

Continuing his role of a middleman, the priest would interpret the Pythia's response for the questioner. (Powell 172)

The Homeric Hymn to Pythian Apollo is an excellent source for the story of Apollo founding his temple at Delphi and his intention to "welcome the tribes of mankind / who gather here, and tell them / most important of all, / what [his] will is," but it makes no mention of the Pythia (161-81). Theognis is the first to mention the Pythia, and the second is Aeschylus in his play Eumenides (Fontenrose 204). The first two lines of Eumenides open with the prophetess's reference to Gaia as the original god of the temple at Delphi.

Many scholars offer evidence to support the idea that the Pythia was an office originating in the cult of Gaia. Dempsey states that the office of the Pythia was always held by a female (originally a virgin, but later at least fifty years old and married) and he points out the connection between the Pythia's gender and the cult of Mother Earth (53-55). Dempsey also points out that the ecstatic nature of the Pythia's prophecy was an abundant characteristic in the cult of Gaia (53-5). A detailed account of the frenzy or mania of the Pythia is presented when Appius Claudius Pulcher visits the oracle at Delphi in Lucan's Civil War (5.64-236). Additionally, many scholars believe that the Python's death at the hand of Apollo symbolized the change in oracles at Delphi (Powell 172)."

Source
http://www.pantheon.org/articles/p/pythia.html

"The Pythia was the priestess who held court at Pytho, the sanctuary of the Delphinians, a sanctuary dedicated to the Greek god Apollo. Pythia were highly-regarded, for it was believed that she channeled prophecies from Apollo himself, while steeped in a dreamlike trance. Originally the god was channeled only once a year, but at the height of its popularity up to three Pythiai were known to hold office.

The sanctuary at Delphi was constructed in the 8th century BC, and the final prophecy given around AD 393, after the Roman Emperor Theodosius ordered the closure of all pagan sanctuaries.

A Pythia was chosen among the priestesses of the temple upon the death of the previous Pythia. Moral character was of utmost importance, and even if the newly-chosen Pythia was married and had a family, she had to relinquish all familial duties in order to fill her role in the temple. Pythias were likely women from higher-class families, were educated, and well-read.

The practice of interpreting the word of Apollo entailed that the Pythia bathe in the Castalian Spring, which was followed by the sacrifice of a goat. She then descended into a special chamber called an adyton beneath the temple which was fumigated with barley meal and laurel leaves on a burning hestia.

There, at the temple center, the Omphalos, she sat on a covered tripod cauldron over a deep well-like chasm. Seated in this way, enveloped by vapors while shaking bay branches, the Pythia would fall into a trance state and channel the god. In this way did the Pythia pronounce judgement and prophecy to those in attendance. Those seeking the counsel of Apollo and his priestess would bring offerings of laurel branches, gifts of money, and a sacrifice of a black ram.

It is a Hellenic tradition that the Pythia entered her trance through the influence of volcanic fumes or hallucinogenic gases emerging from a crevice in the floor of the Castalian Spring.

Although the sanctuary lies directly above two geological fault lines and the spring near the sanctuary contains ethylene, which can provoke  hallucinations, there has been no conclusive determination on whether the trance states of the Pythiai were induced through exposure or was self-induced. "

http://www.ancient.eu.com/Pythia/