A territorial abbey (or territorial abbacy) is a particular church of the Catholic Church comprising defined territory which is not part of a diocese but surrounds an abbey or monastery whose abbot or superior functions as ordinary for all Catholics and parishes in the territory. Such an abbot is called a territorial abbot or abbot nullius dioeceseos (abbreviated abbot nullius and Latin for "abbot of no diocese"). A territorial abbot thus differs from an ordinary abbot, who exercises authority only within the monastery's walls or to monks or canons who have taken their vows there. A territorial abbot is equivalent to a diocesan bishop in Catholic canon law.
The practice arose in part because abbeys served the spiritual needs of Catholics who lived near the monastery, especially in mission territories. The monastery's own chapel was a space of public worship for the laity who had settled nearby, and the monks could also serve as parish clergy in churches near the monastery. The abbot of the monastery, although having received only the priesthood in the sacrament of Holy Orders, was invested with the same administrative authority under canon law as a diocesan bishop for a given territory around the abbey. Thus, with the exception of conferring ordination on priests, the territorial abbot could do almost everything a diocesan bishop would for those under his care, including incardinate (that is, enroll under his jurisdiction) even non-monastic priests and deacons for service in parishes. The territorial abbot, like other abbots and archabbots, had the right to use an episcopal coat of arms and to wear the mitre, the crosier, the ring, and the pectoral cross; if the abbot had been ordained to the episcopacy (very rare), he had the power to ordain his religious who were candidates to the transitional and permanent diaconate and to the priesthood, though they usually had to attend a seminary house of formation located outside the abbot's territory.
Though territorial (like other) abbots are elected by the monks of their abbey, a territorial abbot can only receive the abbatial blessing and be installed under mandate from the pope, just as a bishop cannot be ordained and installed as ordinary of a diocese without such a mandate.
After the Second Vatican Council, more emphasis has been placed on the unique nature of the episcopacy and on the traditional organization of the church into dioceses under bishops. As such, abbeys nullius have been phased out in favor of the erection of new dioceses or the absorption of the territory into an existing diocese. A few ancient abbeys nullius still exist in Europe, and one in Korea.
There are eleven remaining territorial abbeys, as listed by the Vatican in the Annuario Pontificio:
The following abbeys were previously territorial abbeys that were subsequently united with neighbouring dioceses:
Historically there have been more, such as :