Pythia was the name of the high priestess of the Temple of Apollo at Delphi. She specifically served as its oracle and was known as the Oracle of Delphi. Her title was also historically glossed in English as the Pythoness.
The omphalos in the museum of Delphi
View of Delphi with Sacrificial Procession by Claude Lorrain
Modern photograph of the ruins of the Temple of Apollo at Delphi
Priestess of Delphi (1891) by John Collier, showing the Pythia sitting on a tripod with vapor rising from a crack in the earth beneath her
The term "high priest" usually refers either to an individual who holds the office of ruler-priest, or to one who is the head of a religious organisation.
Pinedjem II as High Priest of Amun in Thebes. From his Book of the Dead.
Depiction of a high priest in biblical costume, end of the 17th century, orientalising representation with turban, in the collection of the Jewish Museum of Switzerland.