In a general sense, a person deputed or authorized to perform the functions of another; a substitute in office. The pope pretends to be vicar of Jesus Christ on earth. He has under him a grand vicar, who is a cardinal, and whose jurisdiction extends over all priests, regular and secular.

In the canon law, the priest of a parish, the predial tithes of which are impropriated or appropriated, that is, belong to a chapter or religious house, or to a layman, who receives them, and only allows the vicar the smaller tithes or a salary. Apostolical vicars, are those who perform the functions of the pope in churches or provinces committed to their direction.