An archdeacon is a senior cleric with administrative oversight of part of a diocese under the bishop, and the office has shifted greatly in its weight across the centuries.
Oversight under the bishop
The archdeacon exercises, on behalf of the bishop, a practical oversight of a part of the diocese, the archdeaconry, attending to the administration, the discipline of the clergy, the care of the church buildings and the good order of the parishes, so the office is chiefly administrative and supervisory, the archdeacon acting as an officer of the bishop in the running of the diocese. The title reflects an origin among the deacons, the archdeacon having once been the chief of the deacons who assisted the bishop.
From power to administration
In the early and medieval church the archdeacon grew into a figure of considerable power, exercising real jurisdiction and discipline to the point that the archdeacons became known for the extent of their authority, but the office was later curtailed, and in its modern form, in the Church of England and other churches, it is a senior administrative post held by an experienced priest rather than a seat of great independent power. Entry is by appointment by the bishop. The office survives in the Anglican churches, historically in the Roman Catholic Church, and in Eastern Orthodoxy in a different form. The archdeacon thus represents the administrative deputy of the bishop for a portion of the diocese, an office once powerful and now chiefly one of oversight and good order under the authority of the bishop.