An auxiliary bishop, in the Roman Catholic Church

Roman Catholic Church

The Catholic Church, also known as the Roman Catholic Church, is the world's largest Christian church, with over a billion members. Led by the Pope, it defines its mission as spreading the gospel of Jesus Christ, administering the sacraments and exercising charity...

, is an additional bishop

Bishop (Catholic Church)

In the Catholic Church, a bishop is an ordained minister who holds the fullness of the sacrament of Holy Orders and is responsible for teaching the Catholic faith and ruling the Church....

 assigned to a diocese

Diocese

A diocese is the district or see under the supervision of a bishop. It is divided into parishes.An archdiocese is more significant than a diocese. An archdiocese is presided over by an archbishop whose see may have or had importance due to size or historical significance...

 because the diocesan bishop is unable to perform his functions, the diocese is so extensive that it requires more than one bishop to administer, or the diocese is attached to a royal or imperial office requiring the diocesan bishop's protracted presence at court. According to Canon law

Canon law (Catholic Church)

The canon law of the Catholic Church, is a fully developed legal system, with all the necessary elements: courts, lawyers, judges, a fully articulated legal code and principles of legal interpretation. It lacks the necessary binding force present in most modern day legal systems. The academic...

, no bishop can be ordained

Ordination

In general religious use, ordination is the process by which individuals are consecrated, that is, set apart as clergy to perform various religious rites and ceremonies. The process and ceremonies of ordination itself varies by religion and denomination. One who is in preparation for, or who is...

 without title to a certain and distinct episcopal see

Episcopal See

An episcopal see is, in the original sense, the official seat of a bishop. This seat, which is also referred to as the bishop's cathedra, is placed in the bishop's principal church, which is therefore called the bishop's cathedral...

 which he governs either actually or potentially, therefore auxiliary bishops are titular bishop

Titular bishop

A titular bishop in various churches is a bishop who is not in charge of a diocese.By definition a bishop is an "overseer" of a community of the faithful, so when a priest is ordained a bishop the tradition of the Roman Catholic and Orthodox churches is that he be ordained for a specific place...

s to sees that no longer exist

Titular see

A titular see in various churches is an episcopal see of a former diocese that no longer functions, sometimes called a "dead diocese". The ordinary or hierarch of such a see may be styled a "titular bishop", "titular metropolitan", or "titular archbishop"....

.

Canon law requires that the diocesan bishop appoint each auxiliary bishop as vicar general

Vicar general

A vicar general is the principal deputy of the bishop of a diocese for the exercise of administrative authority. As vicar of the bishop, the vicar general exercises the bishop's ordinary executive power over the entire diocese and, thus, is the highest official in a diocese or other particular...

or episcopal vicar of the diocese.