Bishop in partibus

A titular see in various churches is an episcopal see of a former diocese that no longer functions, sometimes called a "dead diocese".

The ordinary or hierarch of such a see may be styled a "titular metropolitan" (highest rank), "titular archbishop" (intermediary rank) or "titular bishop" (lowest rank), which normally goes by the status conferred on the titular see.

The term is used to signify a diocese that no longer functionally exists, often because the diocese once flourished but the territory was conquered by Muslims or no longer functions because of a schism. The Greek–Turkish population exchange of 1923 also contributed to titular bishoprics. The see of Maximianoupolis was destroyed along with the town that shared its name by the Bulgarians under Emperor Kaloyan in 1207; the town and the see were under the control of the Latin Empire, which took Constantinople during the Fourth Crusade in 1204. Parthenia, in north Africa, was abandoned and swallowed by desert sand.

Titular sees are also used to avoid causing offense or confusion when a bishop of one church serves its faithful in a place where he states no claim of jurisdiction over the faithful of another church dominant there.

During the Muslim conquests of the Middle East and North Africa, some bishops fled to Christian-ruled areas. Even if they did not return and the Christian population of their dioceses dispersed or adopted Islam, they are seen as bishops of those dioceses, who could give rise, even after long interruption (exile and/or vacancy), to a 'restored' line of apostolic succession on each see.

...
Wikipedia