Auxiliary bishop

An auxiliary bishop, in the Roman Catholic Church, is an additional bishop assigned to a diocese 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, no bishop can be ordained without title to a certain and distinct episcopal see which he governs either actually or potentially, therefore auxilary bishops are titular bishops to sees that no longer exist.

Reference

• Catholic Encyclopedia - Auxiliary Bishop