An archdeacon is a senior clergy position in various Christian denominations, including the Church of the East, Chaldean Catholic Church, Syriac Orthodox Church, Anglican Communion, St Thomas Christians, Eastern Orthodox churches, and some other Christian denominations. The Oxford Dictionary of the Christian Church defines an archdeacon as "A cleric having a defined administrative authority delegated to him by the bishop in the whole or part of the diocese". The duties of an archdeacon may include preaching, supervising the deacons and their work, supervising the distribution of alms, and administrative and disciplinary work of the diocese.

In the Roman Catholic Church, the post of archdeacon was once an ordained deacon (rather than a priest) and was one of great importance as a senior official of a diocese. However, the duties are now performed by officials such as auxiliary or coadjutor bishops, the vicar general, and the episcopal vicars.

In the Eastern Christian churches (Eastern Orthodox Church and Eastern Catholic Churches), an archdeacon is the senior deacon within a diocese and has a defined administrative authority delegated to him by the bishop in the whole or part of the diocese.

In the Anglican Communion, an archdeacon is a senior clergy position, and the duties may include assisting the bishop in the administration of the diocese, supervising the work of the clergy, and serving as a liaison between the bishop and the clergy.

In summary, an archdeacon is a senior clergy position with defined administrative authority delegated by the bishop in the whole or part of the diocese. The duties of an archdeacon may vary depending on the denomination.