The pontifex maximus was the chief high priest of the College of Pontiffs in ancient Rome. This was the most important position in the ancient Roman religion, open only to patricians until 254 BC, when a plebeian first held this position. Although in fact the most powerful office in the Roman priesthood, the pontifex maximus was officially ranked fifth in the ranking of the highest Roman priests, behind the rex sacrorum and the flamines maiores.
Augustus as pontifex maximus (Via Labicana Augustus)
Pontifex maximus and Augustus Marcus Aurelius (r. 161–180) performing sacrifice in a relief from the Arch of Marcus Aurelius (Capitoline Museums)
Engraving by Christoph Weigel the Elder of Pope Clement XI, giving him the title pontifex maximus
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.