(Latin papa, from Greek pappas, “father”), the title, since about the 9th century, of the bishop of Rome, the head of the Roman Catholic church. It was formerly given, especially from the 3rd to the 5th century, to any bishop and sometimes to simple priests as an ecclesiastical title expressing affectionate respect. It is still used in the East for the Orthodox (Eastern Orthodoxy) patriarch of Alexandria and for Orthodox priests. (See also papacy.)

The Annuario Pontificio, the official directory of the Holy See, describes the office of the pope by the following titles: Bishop of Rome, Vicar of Jesus Christ, Successor of the Prince of the Apostles, Supreme Pontiff of the Universal Church, Patriarch of the West, Primate of Italy, Metropolitan Archbishop of the Province of Rome, Sovereign of the State of Vatican City, Servant of the Servants of God. The title pope or papa (abbreviated PP.) is officially used only as a less solemn style.

Doctrinally, in Catholic churches, the pope is regarded as the successor of St. Peter (Peter the Apostle, Saint), who was head of the Apostles. The pope, as bishop of Rome, is thus seen to have full and supreme power of jurisdiction over the universal church in matters of faith and morals, as well as in church discipline and government. The twofold basis of this doctrine of papal primacy is the place of Peter (Petrine theory) in the New Testament (in which there are various metaphors expressing his prerogatives) and the place of the Roman church in history. The understanding of papal primacy developed as the church developed, two notable factors being the role of Rome as the imperial city until the 5th century and the religious and political role of the bishop of Rome afterward.

The teaching of the Second Vatican Council (Vatican Council, Second) (1962–65) on the role of bishops counterbalanced the emphasis on papal prerogatives while maintaining the view that the authority of the bishops as a body cannot be separated from that of the pope as its head. Although the Eastern Orthodox have long been willing to give the bishop of Rome the primacy of honour accorded to patriarchs, and although many Protestants have appreciated the moral leadership shown by some recent popes, the Catholic doctrine of papal primacy was still a major obstacle to ecumenical efforts that began in the 20th century.