An apostolic administrator in the Catholic Church is a prelate appointed by the Pope to serve as the ordinary for an apostolic administration. An apostolic administration can either be an area that is not yet a diocese (a stable 'pre-diocesan', usually missionary apostolic administration) or for a diocese, eparchy or similar permanent ordinariate (such a territorial prelature or a territorial abbacy) that either has no bishop (an apostolic administrator sede vacante, as after an episcopal death or resignation) or, in very rare cases, has an incapacitated bishop (apostolic administrator sede plena).
Apostolic administrators of stable administrations are equivalent in canon law with diocesan bishops, meaning they have essentially the same authority as a diocesan bishop. This type of apostolic administrator is usually the bishop of a titular see.
Administrators sede vacante or sede plena only serve in their role until a newly chosen diocesan bishop takes possession of the diocese. They are restricted by canon law in what they can do to the diocese they temporarily administer. For example, such an administrator may not sell real estate owned by the diocese. This type of administrator is commonly an auxiliary bishop of the diocese, a priest serving as the vicar general of the diocese, or the ordinary of a neighboring diocese.
Normally when a diocese falls vacant, either the previously appointed Coadjutor bishop takes possession of the see or (lacking such successor) a vicar capitular/diocesan administrator is chosen locally, but the Pope, having full governmental power, can preempt this choice and name an apostolic administrator instead. Sometimes a retiring, promoted or transferred (arch)bishop is designated to be apostolic administrator until his successor is designated and takes office, sometimes the Metropolitan or a fellow suffragan is appointed.
In May 2015, there were the following stable apostolic administrations, most administered by a (titular or external) bishop.
Most are of the Roman rite; most are in former or current communist countries.
The only exception, belonging to a Uniate, Eastern Catholic rite, is the following :
The Diocese of Pyongyang, North Korea: its last official bishop, Francis Hong Yong-ho, was imprisoned by the communist regime of Kim Il-sung in 1949 and later disappeared. The Metropolitan Archbishop of Seoul (South Korea) acts as the Apostolic Administrator in Pyongyang, as religion is suppressed in North Korea.
In addition, the Personal Apostolic Administration of Saint John Mary Vianney is a non-territorial jurisdiction, similar to a personal prelature, which is exempt, i.e. immediately subject to the Holy See, not part of any ecclesiastical province. It is a separate particular church for traditionalist Catholics within the Brazilian Diocese of Campos, a suffragan of the Metropolitan Archdiocese of Niterói.
The personal apostolic administration was formed by Pope John Paul II to administer to a group of traditionalist Catholic priests, using the Tridentine Mass, who reconciled with Rome on January 18, 2002. The group had been formed by bishop Antônio de Castro Mayer and had been associated with the Society of St. Pius X of Archbishop Marcel Lefebvre.
It has become not unusual that the Pope appoints an Apostolic Administrator to a diocese which is sede vacante, thus replacing the diocesan administrator. Usually, the emeritus bishop will be appointed in such a case. Recently (and in exception to the latter), the archdiocese St. Andrews and Edinburgh had Philip Tartaglia under this procedure.