A dean, in a church context, is a cleric holding certain positions of authority within a religious hierarchy. The title is used mainly in the Anglican Communion, the Roman Catholic Church, and many Lutheran denominations. A dean's assistant is called a subdean.
Latin decanus has various meanings related to the most important of ten parts, perhaps originally in astrology. In the military, it was the head of a group of ten, and by the 5th century CE, it was the head of a group of ten monks. It came to refer to various civil functionaries in the later Roman Empire.
Based on that use, deans in universities now fill various administrative positions.
In the Roman Catholic Church, the Dean of the College of Cardinals and the Cardinal Vice-Dean are the president and vice-president of the college. Both are elected. Except for presiding and delegating administrative tasks, they have no authority over the cardinals, acting as primus inter pares (first among equals).
In the universities that grew out of the cathedral and monastery schools, a university dean came to be an official with various administrative duties.
In the Church of England and elsewhere in the Anglican Communion, the dean is the chief resident cleric of a cathedral or other collegiate church and the head of the chapter of canons. If the cathedral or collegiate church has its own parish, the dean is usually also rector of the parish. However, in the Church of Ireland the roles are often separated, and most cathedrals in the Church of England do not have associated parishes. In the Church in Wales, however, most cathedrals are parish churches, and their deans are now also vicars of their parishes.
In some parts of the Communion (particularly in the Scottish Episcopal Church and, formerly in some cathedrals in England), the senior resident cleric in a cathedral is a provost. Each diocese of the Scottish Episcopal Church has a dean of the diocese: this is a cleric who, rather than heading the cathedral staff, assists the bishop in the administration of the diocese. In this way, a Scottish Episcopal dean is similar to an archdeacon in the other member churches of the Communion (a post that does not exist in the Scottish church). In the Anglican Church of Canada, the roles of senior cleric of the cathedral are combined in one person who is referred to as "Rector of Cathedral and Dean of Diocese". Thus, Peter Elliott is Rector of Christ Church Cathedral and Dean of New Westminster.
The style The Very Reverend distinguishes a cleric as a dean (or a cathedral provost). For example, the Very Reverend June Osborne was Dean of Salisbury Cathedral. The legal act by which a cathedral dean in the Church of England takes up her role is her institution, which is invariably followed in the same service by her installation (into her stall in the cathedral church); an "institution and installation" are very often referred to simply as an installation. In consideration of the high status of a Dean, the Very Reverend title is normally a permanent title preferment.
Some important deans include the deans of St Paul's, Canterbury Cathedral and Westminster Abbey. Westminster Abbey is a royal peculiar, not the seat of any bishop or a cathedral, but is led by a dean. The deans of Washington National Cathedral and St. Patrick's Cathedral, Dublin are likewise important clerics in their churches.
Rural or area deaneries
In many parts of the Anglican Communion, parishes are grouped together to form deaneries, each being a constituent administrative district of the diocese. Usually, a deanery is led by one of the incumbents of the deanery's parishes, who is called a rural dean, but in more urban areas this has often been replaced by the title area dean or regional dean. Such a dean chairs the meeting of the deanery's clergy (which, like a cathedral, is called a chapter), and may also chair a deanery synod. Rural Deans (and those known by alternative titles) rank as primi inter pares of their chapters, and do not have the seniority of cathedral or diocesan deans.
The title "dean" is conferred upon a pastor of a parish who serves as a senior figure, though usually without specific jurisdictional authority, over a section of a diocese. These are sometimes referred to as "rural deans", and are expected to show a degree of leadership among the pastors of the region, known as a deanery. This function is sometimes titled "vicar forane". An episcopal vicar serves a similar function, but has more formal authority and specific powers under canon law.
In recent years, the Catholic Church in England and Wales has introduced the custom of designating cathedral deans, formerly known as cathedral administrators. However, the term differs slightly from the Anglican usage as Catholic deans do not necessarily preside over the cathedral chapter (this function belonging to the office of Provost), and are not necessarily required even to be a member of the chapter. More commonly, in places throughout the world where a cathedral chapter has not been erected (as for instance, in the United States, where there are no chapters at all), the term rector is used for the priest who serves as chief administrator of a cathedral church.
Another important use of the term within the Catholic Church is in the office of the Dean of the College of Cardinals, who serves as senior member of that body. Cardinal Angelo Sodano is the current dean.
Within the Lutheran tradition, particularly in the Nordic and Baltic tradition of evangelical episcopal Lutheranism, senior clergy bear the title 'Dean'. Each diocese usually has a cathedral Dean, in charge of the cathedral church, and a series of area deans to supervise the clergy in a given geographical area. In the Evangelical-lutheran church of Finland, there are also deans in charge of leading the administrative work and personnel of the Chapters. 
United Methodist Christians often speak of a "dean" in terms of the dean of the cabinet. Every annual conference has a bishop's cabinet made up of the district superintendents under the bishop's appointment, as well as occasionally a few other conference officials. One of these superintendents is chosen by his or her colleagues to serve as the dean, usually for one year. This dean then has certain administrative and leadership responsibilities, and is accountable to the bishop.
In various other religious denominations, the title "dean" may be used informally in its wider sense of a senior or venerated member of a congregation. The title may also used in its academic sense in parochial schools.