While the term "Pope" (Latin: papa "father'") is used in several churches to denote their high spiritual leaders, this title in English usage generally refers to the supreme earthly head of the Roman Catholic Church. The title itself has been used officially by the head of the Roman Catholic Church since the tenure of Pope Siricius.
There is no official list of popes, but the Annuario Pontificio, published every year by the Vatican, contains a list that is generally considered to be the most authoritative. Its list is used here. Several changes have been made in the list during the 20th century. Antipope Christopher was considered legitimate for a long time. Pope-elect Stephen (March 23–March 26, 752), who died of apoplexy three days after his election and before his ordination as a bishop, was considered legitimate under the name Stephen II until the 1961 edition, at which time his name was removed. Although these changes are no longer controversial, a number of modern lists still include this "first Pope Stephen II". It is probable that this is because they are based on the 1913 edition of the Catholic Encyclopaedia, which is in the public domain.
The 10 longest-reigning popes after St. Peter (34 years) whose reign lengths can be determined from contemporary historical data are the following:
The number of calendar days includes part days, e.g., if a pope's reign commenced on 1 August and he died on 2 August, this would count as having reigned for two calendar days.
Peter's reign has been determined from traditional sources, but their accuracy is far from certain. Traditionally, St. Peter is said to have spent twenty five years in Rome. However, at least since the time when Pope Pius IX completed his twenty fifth year as Pope in 1871, thus surpassing the traditional figure for Peter's sojourn in Rome, Peter's term as Pope has been reckoned from the time that Catholics consider Jesus to have bestowed the office upon him. This date cannot be known with any certainty, but is taken to be about 30. To add to the uncertainty, two different death years are proposed for Peter, 64 and 67. A reckoning of Peter as Pope for only the twenty five years he traditionally spent in Rome would make him the fourth-longest-reigning pope. However, his papal office taken as a total of his leadership over the Church would have him having been Pope for 31-34 years.
Many non-Catholic Christians dispute St. Peter being on any list of popes at all, as there is no proof of any Bishop of Rome claiming the papal title until centuries after the death of Jesus. Further, some hold that Peter could not simultaneously be a bishop and an apostle.
Roman Catholics, on the other hand, consider St. Peter necessarily to be the first pope by virtue of his reputed commission by Jesus (Matthew 16:18, "And I say to thee: That thou art Peter; and upon this rock I will build my church, and the gates of hell shall not prevail against it.") and because they believe he was the first Bishop of Rome. The 1953 discovery of bones belonging to an aged man of Semitic origin, buried underneath the altar of St. Peter's Basilica, according to legend, the traditional burial spot of the apostle Peter, is presented by Catholics as the most plausible proof that Peter lived in and was martyred in Rome. By Catholic understanding, all later popes reign by virtue of their succession to St. Peter in his office.
Another argument is the definition of the Pope which is the Successor of St. Peter. Originally, popes were styled "Vicar of Peter," before this was changed to "Vicar of Christ," which also seems to suggest that the pontiff is, by definition, a representative or successor of Peter. As by definition Peter could not be a successor to himself, this is used by those disposed towards rejecting him as a Pope as grounds for his disqualification for that title.
For more discussion of disputes about the nature of papacy, and of the evolution of the term pope, see pope.