A bishop is an ordained member of the clergy who is entrusted with a position of authority and oversight in a religious institution. In Christianity, bishops are normally responsible for the governance and administration of dioceses. The role or office of the bishop is called episcopacy or the episcopate. Organisationally, several Christian denominations utilise ecclesiastical structures that call for the position of bishops, while other denominations have dispensed with this office, seeing it as a symbol of power. Bishops have also exercised political authority within their dioceses.

Traditionally, bishops claim apostolic succession and the historic episcopacy, a direct historical lineage dating back to the original Twelve Apostles or Saint Paul. The bishops are by doctrine understood as those who possess the full priesthood given by Jesus Christ, and therefore may ordain other clergy, including other bishops. A person ordained as a deacon, priest (i.e. presbyter), and then bishop is understood to hold the fullness of the ministerial priesthood, given responsibility by Christ to govern, teach and sanctify the Body of Christ (the Christian Church). Priests, deacons and lay ministers co-operate and assist their bishops in pastoral ministry.

Some Pentecostal and other Protestant denominations have bishops who oversee congregations, though they do not necessarily claim apostolic succession, with exception to those Pentecostals and Charismatics affiliated to churches founded by J. Delano Ellis and Paul S. Morton.

The English word bishop derives, via Latin episcopus, Old English biscop, and Middle English bisshop, from the Greek word ἐπίσκοπος, epískopos, meaning "overseer" or "supervisor". Greek was the language of the early Christian church, but the term epískopos did not originate in Christianity: it had been used in Greek for several centuries before the advent of Christianity.

The English words priest and presbyter both derive, via Latin, from the Greek word πρεσβύτερος, presbýteros, meaning "elder" or "senior", and not originally referring to priesthood.

In the early Christian era the two terms were not always clearly distinguished, but epískopos is used in the sense of the order or office of bishop, distinct from that of presbýteros, in the writings attributed to Ignatius of Antioch in the second century.

The earliest organization of the Church in Jerusalem was, according to most scholars, similar to that of Jewish synagogues, but it had a council or college of ordained presbyters (πρεσβύτεροι, 'elders'). In Acts 11:30 and Acts 15:22, a collegiate system of government in Jerusalem is chaired by James the Just, according to tradition the first bishop of the city. In Acts 14:23, the Apostle Paul ordains presbyters in churches in Anatolia. The word presbyter was not yet distinguished from overseer (ἐπίσκοπος, episkopos, later used exclusively to mean bishop), as in Acts 20:17, Titus 1:5–7 and 1 Peter 5:1. The earliest writings of the Apostolic Fathers, the Didache and the First Epistle of Clement, for example, show the church used two terms for local church offices—presbyters (seen by many as an interchangeable term with episkopos or overseer) and deacon.

In the First Epistle to Timothy and Epistle to Titus in the New Testament a more clearly defined episcopate can be seen. Both letters state that Paul had left Timothy in Ephesus and Titus in Crete to oversee the local church. Paul commands Titus to ordain presbyters/bishops and to exercise general oversight. John Zizioulas argues that "The task of the Bishop was from the beginning principally liturgical, consisting in the offering of the Divine Eucharist." The authorship of both those letters is questioned by many scholars in the field and the question whether they reflect a first or second century structure of church hierarchy is among the arguments used in the debate as to their authenticity.