In the broadest sense, a vicar is anyone who is acting as a substitute or agent for a superior. In this sense, the title is comparable to lieutenant. Usually the title appears in a number of Christian ecclesiastical contexts.
The Pope uses the title Vicarius Christi, meaning, the vicar of Jesus Christ. They first used this title in the eighth century; earlier they used the title vicar of St. Peter or vicarius principis apostolorum, the vicar of the chief of the apostles.
Some papal legates are honoured by the title Vicar of the Apostolic See.
In the Anglican communion, vicar is the ordinary title given to certain parish priests. Historically, Anglican parish clergymen were divided into rectors, vicars and perpetual curates. These were distinguished according to the way in which they were remunerated. The church was supported by tithes - taxes (traditionally of 10%) levied on the agricultural output of the parish. These were divided into greater tithes levied on wheat, hay and wood, and lesser tithes levied on the remainder. A rector received both greater and lesser tithes, a vicar the lesser tithes only. A perpetual curate received no tithe income and was supported by the diocese. The adjective perpetual emphasises that such a clergyman enjoyed the same security of tenure as his more affluent peers. An Act of Parliament of 1868 permitted perpetual curates to style themselves vicars. The conjunction of this change with near-contemporaneous church reforms aimed at reducing the disparities of income among clergymen meant that the distinction between the grades of clergymen became progressively less relevant and remarked upon.
In either tradition, a vicar can be the priest of a chapel, a church which is not a parish church.