A territory which is set apart from any diocese and in which the clergy and the people are subject to an abbot as their local ordinary. The prelate who presides over abbacy nullius is called an abbot nullius.

The origin of the abbacies nullius goes back to the celebrated monasteries, which since 9th and 10th centuries were exercising pastoral care on the neighbouring people. They were gradually exempted from the bishops and the abbots obtained quasi-Episcopal jurisdiction. The popes from 11th and 12th centuries confirmed such exemption and gave the abbots the jus pontificalium. Since abbacy nullius were few, they were not mentioned in the Decretals of Gregory IX. However, we can find hints of it in the writings of Alexander IV (see for instance c. 3, 5, 7, VI).

The 1917 Code distinguished two types of abbacy nullius: abbacy nullius that consists of three or more parishes and abbacy nullius that consisted of less than three parishes. The former was directed by the canons of the Code, while the latter is directed by special laws (CIC, 1917, c. 319). The same Code sets out in canons 207-209 certain rights and privileges to the abbot nullius. Even though they were not consecrated bishops they had the right to consecrate churches, immovable altars, impart all blessings reserved to the bishops, except the pontifical blessing within their territory. They also could give confirmation, first tonsure, and minor orders. They in their territory could use insignia of a bishop with throne and canopy. Outside his territory they could wear pectoral cross, the right to wear the ring with the gem and purple cap.

The 1983 Code does not mention this type of ecclesiastical circumspition.