An Apostolic Vicariate is a form of territorial jurisdiction of the Catholic Church established in missionary regions and countries which do not have a Diocese. It is therefore essentially provisional, though it may last for a century or more. The ultimate hope is that the region will generate sufficient number of Catholics and become stable enough for the Church as to warrant erection as a Diocese.
An Apostolic Vicariate is led by a Vicar Apostolic who is usually a Titular Bishop. While such a territory can be classed as particular Church, according to Canon 371.1 of the Latin Code of Canon Law a Vicar Apostolic’s jurisdiction is an exercise of the Jurisdiction of the Pope — that is to say, the territory comes directly under the Pope as “Universal Bishop,” and the Pope exercises this authority through a “Vicar” or Delegate. This is unlike the jurisdiction of a Diocesan Bishop; whose jurisdiction derives directly from his Office.
Like any Ecclesiastical Jurisdiction, an Apostolic Vicariate may be administered by the Bishop of a neighbouring Diocese, or even by a Priest appointed transitionally as an Apostolic Administrator. As in a regular Diocese, the Vicar Apostolic may appoint Priests as Vicars exercising limited jurisdiction over the Apostolic Vicariate.
An Apostolic Vicariate is to be distinguished from an Apostolic Prefecture, a similar type of Territory that is usually under the leadership of a Priest, which is not yet organised enough to be termed an Apostolic Vicariate. The Apostolic Vicariate is also to be distinguished from a territorial abbacy (or “abbey nullius”) — an area not a Diocese but under the direction of the Abbot of a Monastery.
The usual sequence of development is: Mission, Apostolic Prefecture, Apostolic Vicariate and finally Diocese.