What do we call a church leader in charge of a region (diocese)?

The story behind the answer

A diocese is the basic territorial unit of church governance, and its leader oversees clergy, sacraments, and doctrine within that region. By contrast, a pastor leads a single parish, an archbishop supervises several dioceses within a province, and a cardinal is a senior advisor to the pope and papal elector rather than a territorial governor.

In the Middle Ages, these leaders were not only spiritual authorities but often powerful political figures. Many controlled vast lands, collected tithes, and in the Holy Roman Empire some ruled as “prince-bishops.” The clash over who had the right to appoint them—kings or popes—sparked the Investiture Controversy. Their cathedra (bishop’s seat) in the cathedral symbolized teaching authority, and their crozier signified pastoral care. Diocesan schools and scriptoria helped preserve and transmit learning throughout the medieval period.