The Pontifex Maximus (Latin, literally: "greatest pontiff") was the high priest of the College of Pontiffs (Collegium Pontificum) 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 occupied this post. A distinctly religious office under the early Roman Republic, it gradually became politicized until, beginning with Augustus, it was subsumed into the Imperial office. Its last use with reference to the emperors is in inscriptions of Gratian (reigned 375–383) who, however, then decided to omit the words "pontifex maximus" from his title.
The word "pontifex" later became a term used for Christian bishops, including the Bishop of Rome, and the title of "Pontifex Maximus" was applied within the Roman Catholic Church to the Pope as its chief bishop. It is not included in the Pope's official titles, but appears on buildings, monuments and coins of popes of Renaissance and modern times.
Read more about Pontifex Maximus: Etymology, Origins in The Regal Period, Roman Republic, Roman Empire, Roman Catholic Use of The Title, Tradition of Sovereign As High Priest, Incomplete List of Pontifices Maximi, In Popular Culture