In the Christian Church of the Middle Ages, a bishop was the chief pastor of a diocese; that is, an area containing more than one congregation. The bishop was an ordained priest who served as pastor of one congregation and oversaw the administration of any others in his district.
Any church that served as the primary office of a bishop was considered his seat, or cathedra, and was therefore known as a cathedral. The office or rank of a bishop is known as a bishopric.
Origins of the term "Bishop"
The word "Bishop" derives from the Greek epískopos (ἐπίσκοπος), which meant an overseer, curator or guardian.
Like any priest, a bishop baptized, performed weddings, gave last rites, settled disputes, and heard confession and absolved. In addition, bishops controlled church finances, ordained priests, assigned clergy to their posts, and dealt with any number of matters pertaining to Church business.
Types of Bishops in Medieval Times
An archbishop was a bishop who oversaw several dioceses besides his own. The term "metropolitan" has sometimes been used for the archbishop of a city.
The Pope is the bishop of Rome. The holder of this see was considered the successor to St. Peter, and the office grew in prestige and influence over the first few centuries of the Middle Ages. Before the end of the fifth century, the office was established as the foremost authority in the western Christian Church, and the bishop of Rome became known as the father, or papa, or pope.
Patriarchs were bishops of particularly important sees in the eastern churches (which, after the Great Schism of 1054, would eventually become known as the Eastern Orthodox Church). This included the apostolic sees -- those believed to have been founded by Apostles: Alexandria, Antioch, Constantinople, and Jerusalem
Cardinal-bishops (now known simply as cardinals) were a privileged class as far back as the 8th century, and only those bishops who had received the red hat (a mark of a cardinal) could elect the pope or become pope.
Secular Influence as Well as Spiritual Power
Some Christian churches, including the Roman Catholic and Eastern Orthodox, maintain that bishops are the successors of the Apostles; this is known as apostolic succession. As the Middle Ages unfolded, bishops often held secular influence as well as spiritual power thanks in part to this perception of inherited authority.
(article from https://www.learnreligions.com/definition-of-bishop-1788456
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