A prophetess of the god Apollo, who was, in Greek antiquity, charged with interpreting the predictions and advice of the god from his Oracle at Delphi. The Pythia and the oracle are often confused, however an "oracle" is simply a message, delivered to the priestess non-verbally by the god and translated by her for the benefit of the person seeking advice.
The name of the Pythia is likely derived from Python, the giant dragon whom Apollo had slain in order to take over the sanctuary from its original occupant, the earth goddess Ge, or Gaia.
There are varying and sometimes conflicting theories about how much power the Pythia really had and to what degree she was the puppet on the strings of the male priests who served in the temple. The traditional theory was that the Pythia was no more than a figurehead, shrouded in mystery and hidden by smoke and darkness, whose indistinct cries and murmurs the priests would couch in what vague terms most suited to their purpose. However, modern resarch is inclined to favour reports that the Pythia spoke to her supplicants herself, clearly and directly.
The former view of the Pythias' office is based primarily on reports from the historian Herodotus. It is however possible that by the time Herodotus could visit Delphi or speak to someone who has, there would have been a decline in the power of the Oracle, brought about by the general decline of the city-states. Another reasonable hypothesis is that different priestesses reacted differently to the pressures of office, and different generations of priests saw their responsibilities in different lights, making for a fluctuating dynamic of inter-cult power struggles.
Practically nothing is known about the priestesses themselves. They are not recorded by name as far as I can tell and there are no reports as to how they were chosen for office or dedicated. We don't know if they were high-born ladies or peasant women, whether they served until death or only for a term, and if the latter, what became of them after their service was over.