A deacon is a member of the diaconate, an office in Christian churches that is generally associated with service of some kind, but which varies among theological and denominational traditions.

Major Christian denominations, such as the Catholic Church, the Oriental Orthodox Churches, the Eastern Orthodox Church, Lutheranism, Presbyterianism, Methodism, and Anglicanism, view the diaconate as an order of ministry.

Permanent deacons (or distinctive deacons) are those who do not later transition to another form of ministry, in contrast to those continuing their formation who are then often called transitional deacons.

The word deacon is derived from the Greek word diákonos (διάκονος), which is a standard ancient Greek word meaning "servant", "waiter", "minister", or "messenger". Recent research has highlighted the role of the deacon "as a co-operator" and "go-between," emphasizing their intermediary position in early Christian communities.

It is generally assumed that the office of deacon originated in the selection of seven men by the apostles, among them Stephen, to assist with the charitable work of the early church as recorded in Acts of the Apostles chapter 6. Newer research emphasizes that while the deacons' role was indeed to help, their assistance involved more liturgical, teaching and leadership functions than purely charitable work.

The Greek word diakónissa (διακόνισσα), meaning deaconess, is not found in the Bible. However, one woman, Phoebe, is mentioned at Romans 16:1–2 as a deacon or deaconess (διάκονος, diákonos) of the church in Cenchreae. Nothing more specific is said about her duties or authority, although it is assumed she carried Paul's Letter to the Romans.

Female deacons are mentioned by the Roman author Pliny the Younger in a letter to the Roman emperor Trajan dated c. 112:

I believed it was necessary to find out from two female slaves (ex duabus ancillis) who were called deacons (ministrae), what was true—and to find out through torture (per tormenta)