Prince Bishops

A prince-bishop is a bishop who is also the civil ruler of some secular principality and sovereignty. Thus the principality or prince-bishopric ruled politically by a prince-bishop could wholly or largely overlap with his diocesan jurisdiction, since some parts of his diocese, even the city of his residence, could be exempt from his civil rule, obtaining the status of free imperial city. If the episcopal see is an archbishopric, the correct term is prince-archbishop; the equivalent in the regular (monastic) clergy is prince-abbot. A prince-bishop is usually considered an elected monarch.

In the West, with the decline of imperial power from the 4th century onwards in the face of the barbarian invasions, sometimes Christian bishops of cities took the place of the Roman commander, made secular decisions for the city and led their own troops when necessary. Later relations between a prince-bishop and the burghers were invariably not cordial. As cities demanded charters from emperors, kings, or their prince-bishops and declared themselves independent of the secular territorial magnates, friction intensified between burghers and bishops.

In the Byzantine Empire, the still Emperors passed general legal measures assigning all bishops certain rights and duties in the secular administration of their dioceses, but that was part of a caesaropapist development putting the Eastern Church in the service of the Empire, with its Ecumenical Patriarch almost reduced to the Emperor's minister of religious affairs.

Bishops had been involved in the government of the Frankish realm and subsequent Carolingian Empire frequently as the clerical member of a duo of envoys styled Missus dominicus, but that was an individual mandate, not attached to the see. Prince-bishoprics were most common in the feudally fragmented Holy Roman Empire, where many were formally awarded the rank of an Imperial Prince Reichsfürst, granting them the immediate power over a certain territory and a representation in the Imperial Diet (Reichstag).

...
Wikipedia