Coadjutor bishop
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.