The Pythia was the priestess who held court at the Oracle at Delphi, a sanctuary to the God Greek Apollo. She was highly-regarded, for it was believed that she channeled prophecies from Apollo himself, while steeped in a dreamlike trance. The Oracle 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.
The 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 the Pythia bathing in the Castalian Spring, then descending into her special chamber beneath the temple, where she would sit on a tripod, holding a cauldron of special water and smoldering laurel leaves. 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 believed that the Pythia entered a trance caused by hallucinogenic gases that emerged from a crevice in the floor of the Castalian Spring. The Oracle of Delphi lies directly above two geological fault lines, and the spring near the Oracle contains ethylene, a hallucinogenic substance. It is thus likely that the trance was induced by gases that emerged into the temple room due to its unique geological location.