A canon (from the Latin canonicus, itself derived from the Greek κανωνικος 'relating to a rule') is a priest who is a member of certain bodies of the Christian clergy subject to an ecclesiastical rule (canon).
Originally, a canon was a cleric living with others in a clergyhouse or, later, in one of the houses within the precinct or close of a cathedral and ordering his life according to the orders or rules of the church. This way of life began to become common (and be referred to in manuscripts) in the 8th century. In the 11th century, some churches required clergy thus living together to adopt the rule first proposed by Saint Augustine that they renounce private wealth. Those who embraced this change were known as Augustinian or regular canons, whilst those who did not were known as secular canons.
One of the functions of the cathedral canons in the Roman Catholic Church is to elect a Vicar Capitular (now named a Diocesan Administrator in English) to serve during a sede vacante period of the diocese.
In the Catholic Church the members of the chapter of a cathedral or of a collegiate church (so called after their chapter) are canons. Depending on the title of the church, several languages use specific titles, e.g. in German Domherr in a Dom (i.e. cathedral), Stiftsherr in a prelature that has the status of a Stift (notably under a Prince of the Church).
All canons of the Church of England have been secular since the Reformation, although an individual canon may themselves also be a member of a religious order. Mostly, however, they are ordained (that is, priests or members of the clergy). Today, the system of canons is retained almost exclusively in connection with cathedral churches. A canon is a member of the chapter of (for the most part) priests, headed by a Dean, which is responsible for administering a cathedral or certain other churches that are styled collegiate church. The Dean and Chapter are the formal body which has legal responsibility for the cathedral and for electing the (arch)bishop.
Canon is still given in many dioceses to senior parish priests as a largely honorary title. It is usually awarded in recognition of long and dedicated service to the diocese. Honorary canons are members of the chapter in name but are non-residential and receive no emoluments, but are entitled to call themselves canon and have a role in the administration of the cathedral.
Generally speaking, canons in the Anglican Church or the Episcopal Church are of this sort, and thus are equivalent to a monsignor in the Catholic Church, often wearing the violet or violet-trimmed cassock which is associated with that rank.
In addition to canons who are clerks in holy orders, cathedrals in the Anglican communion may also appoint lay persons as canons. The rank of "lay canon" is especially conferred upon diocesan chancellors (the senior legal officer of the diocese, who is usually, though not exclusively, a lay person).
Canon-Professors at Oxford
§2 of The Church of England (Miscellaneous Provisions) Measure 1995 was passed for the express purpose of enabling Christ Church, Oxford to appoint not more than two Lay Canons. One of the motivations for this provision was the fact that, under the Ecclesiastical Commissioners Act 1840 (§6), the position of Regius Professor of Ecclesiastical History in the University of Oxford was annexed to a Residentiary Canonry of the Cathedral, meaning that the Regius Professorship could be held only by an Anglican priest. Following the death of Peter Hinchliff in 1995 the Regius Professorship was held by Henry Mayr-Harting, a Roman Catholic layman, from 1997 until 2003, and was taken up by another lay person, Sarah Foot, in Michaelmas Term 2007. Three other Statutory Professorships, the Regius Professorship of Divinity, Lady Margaret Professorship of Divinity, and Regius Professorship of Moral and Pastoral Theology, are annexed to canonries of Christ Church and must be held by Anglican priests.
Minor canons are those clergy who are members of the cathedral's establishment and take part in the daily services but are not part of the formal Chapter. These are generally more junior clergy, who in a parish church would be serving a curacy.
Many bishops endeavoured to imitate St. Augustine and St. Eusebius, and to live a common life with the clergy of their Church. Rules taken from the sacred canons were even drawn up for their use, of which the most celebrated is that of St. Chrodegang, Bishop of Metz (766). In the tenth century, this institution declined; the canons, as the clergy attached to a church and living a common life were called, began to live separately; some of them, however resisted this relaxation of discipline, and even added poverty to their common life. This is the origin of the canons regular. Pope Benedict XII by his Constitution "Ad decorem" (15 May, 1339) prescribed a general reform of the canons regular. The Canons Regular ex professo united Holy orders with religious life, and being attached to a church, devoted themselves to promoting the dignity of Divine worship. With monks, Holy orders are accidental and secondary, and are superadded to the religious life; with canons as with the clerks regular, Holy orders are the principal thing, and the religious life is superadded to the Holy orders.