An auxiliary bishop (also called vicar bishop, suffragan bishop, or chorepiscopus) is a bishop with no territorial authority working under the authority of a diocesan bishop. The auxiliary typically is given episcopal functions to assist the work of the diocesan hierarch under whose authority he operates. He typically holds the title of a city within the diocese of his superior, though he is not considered the ruling bishop of that city.
Being given a title which is within the territory of another bishop technically makes auxiliary bishops uncanonical. A number of Orthodox jurisdictions deal with this canonical problem by instead appointing titular bishops, who hold extinct sees which are not within the territory of a ryuling bishop.
In the ancient Church, the chorepiscopus (sometimes called a "country bishop") was like the modern auxiliary bishop but usually served in the countryside near a city which had its own bishop and under whose authority he operated. He did not typically have authority to perform ordinations but could function in other episcopal ways. It is generally believed that the chorepiscopate developed in the part of the Roman Empire which is modern day Romania.