The group of churches that a bishop supervises is known as a diocese. Typically, a diocese is divided into parishes that are each overseen by a priest.
The original dioceses, in ancient Rome, were political rather than religious. Rome was divided into dioceses, each of which was made up of many provinces. After Christianity became the Roman Empire's official religion in the 4th century, the term gradually came to refer to religious districts. The Catholic Church has almost 3,000 dioceses. The Greek root of diocese is dioikesis, "government, administration, or province."