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 bishop", "titular metropolitan", or "titular archbishop".
The term is used to signify a diocese that no longer functionally exists, often because the diocese once flourished but the territory was conquered for Islam by jihad (from the seventh century forward; the Ecumenical Patriarchate of Constantinople, for example, Constantinople being conquered by the Turks in 1453 AD, and now called by them "Istanbul"), or because of a schism. The Greek-Turkish population exchange of 1923 also contributed to titular bishoprics. Not all titular sees came about in this way, such as the see of Maximianoupolis which 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 4th Crusade in 1204.
Titular sees are also used to avoid causing offense or confusion when a bishop of one denomination serves a place which is also the see of a bishop of a different denomination.
Roman Catholic Church 
Until 1882, sees lost to the Muslim conquests were distinguished by the Latin phrase in partibus infidelium, "in the territory of the unbelievers", or more often simply in partibus, or because the diocese was dissolved, absorbed into another diocese, or the see was transferred to another location. These non-functional dioceses, now merely historical names, are called "titular sees."
At one time coadjutor bishops and archbishops were given titular sees, but they are now raised to the diocese or archdiocese that they will oversee as coadjutor. Retired bishops and archbishops were also given titular sees, but the common practice now is to name them Bishop or Archbishop Emeritus of the see they retired from.
While the Vatican hopes that titular sees will one day become active dioceses once again, it realizes in most cases the chances of that happening in the near future are low. Some titular sees appear also to remain vacant for ecumenical reasons (e.g. a number of those in the immediate vicinity of Greek Orthodox patriarchates).
In partibus infidelium 
In partibus infidelium (often shortened to in partibus, or abbreviated as i.p.i.), is a Latin phrase meaning "in the lands of unbelievers," words once added to the name of many of the sees conferred on non-residential or titular Roman Catholic bishops, for example: "John Doe, Bishop of Tyre in partibus infidelium".
During the expansion of Christianity in the early centuries AD, hundreds of dioceses were created in what is now Turkey, in the Middle East and throughout North Africa. In many of these areas, the presence of the Church diminished or disappeared as a result of schism, or as regions were conquered by Muslims. These dioceses were designated as in partibus. In other circumstances, reorganizations would sometimes lead to dioceses being absorbed into one or more other dioceses. At times the see cities of dioceses were relocated to other cities, and the diocese in question was renamed.
An example of an enduring in partibus bishopric is that of the Bishop of Bethlehem. In 1168, the crusading William IV, Count of Nevers had promised the Bishop of Bethlehem that if Bethlehem should fall under Muslim control, he would welcome either him or his successors in the small town of Clamecy in the present day Burgundy, France. After the capture of Bethlehem by Saladin in 1187, the bequest of the by then deceased Count was honoured and the Bishop of Bethlehem duly took up residence in the hospital of Panthenor, Clamecy in 1223. Clamecy remained the continuous 'in partibus infidelium' seat of the Bishopric of Bethlehem for almost 600 years, until the French Revolution in 1789.
Prospero Fagnani (in cap. Episcopalia, i, De privilegiis) says that the regular appointment of titular bishops dates back only to the time of the Fifth Lateran Council under Leo X (Session IX); cardinals alone were authorized to ask for them for the dioceses. St. Pius V extended the privilege to the sees in which it was customary to have auxiliary bishops. Since then the practice became more widespread. The Sacred Congregation of the Propaganda, by its circular letter of 3 March 1882, abolished the expression in partibus infidelium; the present custom is to join to the name of the see that of the district to which it formerly belonged, e.g. "Johannes Doe, Archiepiscopus Corinthius in Achaiâ," or else merely to say "titular bishop".
Occasionally, the transfer of a diocesan bishop to a titular see has been used by the Holy See to strip a bishop of his responsibilities. For instance, in 1995, Bishop Jacques Gaillot, known for his controversial activism on Catholic-sensitive social and political topics, was transferred from the Diocese of Évreux in France to Partenia, a titular see in Algeria.
Orthodox Church 
One reason is to avoid causing offense or confusion when an Orthodox bishop serves a place which is also the see of a bishop of a different jurisdiction: the Orthodox bishop in Oxford, England, is titled Bishop of Diokleia; the Russian Orthodox diocese of the United Kingdom is the Diocese of Sourozh.
See also 
- Kiel, Machiel (1971). "Observations on the History of Northern Greece during the Turkish Rule: Historical and Architectural Description of the Turkish Monuments of Komotini and Serres, their place in the Development of Ottoman Turkish Architecture and their Present Condition". Balkan Studies 12: 417.
- "In Partibus Infidelium". Catholic Encyclopedia. New York: Robert Appleton Company. 1913.
- de Sivry, L: "Dictionnaire de Geographie Ecclesiastique", page 375, 1852 ed, from ecclesiastical record of letters between the Bishops of Bethlehem 'in partibus' to the bishops of Auxerre.
- Demetrius Kiminas, The Ecumenical Patriarchate: A History of Its Metropolitans with Annotated Hierarch Catalogs, 2009. ISBN 1-4344-5876-8.
- Orthodox Wiki
- List of all Titular sees by Giga-Catholic Information