(from Latin vicarius, “substitute”), an official acting in some special way for a superior, primarily an ecclesiastical title in the Christian Church. In the Roman Empire as reorganized by Emperor Diocletian (reigned 284–305), the vicarius was an important official, and the title remained in use for secular officials in the Middle Ages. In the Roman Catholic Church, “vicar of Christ” became the special designation of the popes (pope) starting in the 8th century, and eventually it replaced the older title of “vicar of St. Peter.”

In the early church, the name vicar, or legate, was used for the representative of the pope to the Eastern councils. Beginning in the 4th century, vicar of the apostolic see or vicar apostolic came to mean a residential bishop with certain rights of surveillance over neighbouring bishops. By the 13th century a vicar was an emissary sent from Rome to govern a diocese that was without a bishop or in special difficulties. The Roman Catholic Church in England was governed by vicars apostolic from 1685 until 1850 when Pope Pius IX reestablished the English hierarchy. In modern times vicars apostolic are generally titular bishops appointed to rule territories not yet organized into dioceses.

A vicar general is appointed by the bishop as the highest administrative officer of the diocese, with most of the powers of the bishop. The pope governs his own diocese of Rome through a cardinal vicar and a special vicar general for the Vatican City. Vicar general is also the title for some heads of religious orders.

A vicar forane (or rural dean) is a priest in charge of a subdivision of a diocese called a forane vicariate, or deanery. In canon law a priest working with or in place of the pastor of a parish is called a vicar, or curate.

In the Church of England (England, Church of), a vicar is the priest of a parish the revenues of which belong to another, while he himself receives a stipend. His official place of residence is a vicarage. A vicar general is employed by some bishops to assist in special duties.

In the Protestant Episcopal Church (Episcopal Church in the United States of America) and in some Lutheran (Lutheranism) churches, the vicar is an assistant to the pastor. In Lutheran churches the pastor's assistant is someone who is still in the course of ministerial education.