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 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.
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 never returned, and even if the Christian population of their dioceses dispersed or adopted Islam, they were still seen as bishops of those dioceses.
The term Latin: in partibus infidelium, often shortened to Latin: in partibus or i.p.i., meaning "in the lands of the unbelievers", was added to the name of the see conferred on titular (non-diocesan) Latin Church bishops. Formerly, when bishops were forced to flee from the invading infidels, they were welcomed by other Churches, while preserving their titles and their rights to their own dioceses. They were even entrusted with the administration of vacant sees of other diocese. In later days it was deemed fitting to preserve the memory of ancient Christian Churches that had fallen into the hands of the unbelievers; this was done by giving their names to auxiliary bishops or bishops in missionary countries. These bishops do not reside in the sees whose titles they bear, cannot exercise any power over them, and are not entrusted with their care. They are therefore called titular bishops, as opposed to diocesan bishops, and the sees themselves are called titular sees, as opposed to residential sees.
According to Auguste Boudinhon, in Catholic Encyclopedia, Prospero Fagnani said that the regular appointment of titular bishops dates back only to the time of the Fifth Lateran Council, in 1514; cardinals alone were authorized to ask for them for their dioceses. Pope Pius V extended the privilege to the sees in which it was customary to have auxiliary bishops. Since then the practice became more widespread.
Although the normal constitution of the hierarchy has always been built on the idea of local jurisdiction of the bishops, there are indications, even in the early history of the Church, of many who did not enjoy what is usually called ordinary jurisdiction. Besides those who were endowed with the episcopal character, in order to assist the local bishops, there were those who had been driven from their dioceses by infidels or by heretics, or who for other reasons could not reside in the places to which they had been appointed. The spread of Islam through Muslim conquests in Asia and Africa was responsible for hundreds of abandoned sees. During the Crusades, the Latins, who established new Christian communities, composed of Europeans and belonging to the Latin Rite, procured the erection of new dioceses for their benefit, and these in turn, during the growth of the Ottoman Empire, increased the number of abandoned sees. The final development of the list of sees, called Latin: in partibus infidelium, took shape, at first, from the attempt of the Holy See to keep up the succession of bishops in these dioceses, in the hope of reconquering their territory from the infidel. When all hope of such redemption was given up, these titles were still conferred on those who were chosen to assist the diocesan bishops in their labors. After the 14th century the large increase of population in the great centers rendered such assistance particularly necessary. In the 16th century the Holy See inaugurated the policy of consecrating nuncios and other prelates, delegated to represent the Pope in his relations with the different nations, so that they would be equals with the diocesan bishops of the countries in which they were ambassadors.
The foundation of the Congregation for the Propagation of the Faith, in 1622, gave a great impetus to the missionary work of the Church in China and Japan, and elsewhere, a great increase in the number of bishops became necessary and those received their titles from the ancient abandoned sees.
Only about 1850, was any attempt made to compile a list of such sees. Gaetano Moroni had already, in 1840, began publication of his 103 volume with a separate six volume index.  Moroni acknowledged the great difficulties in compiling this work, even after he thoroughly examined all the sources available to him.
In 1851, the Annuario Pontificio began to have such a list, but it did not purport to be complete. On the contrary, it contained only those that were in general use. Names of dioceses disappeared and were listed again when the titles were actually assigned.
Until 1882, these titles were given as Latin: in partibus infidelium. According to Corrigan, the story goes that King George I of Greece (a Lutheran) complained to Pope Leo XIII that he and his people were injured by this appellation, saying to Leo XIII, "we are not infidels, we are Christians; we are Catholics." Leo XIII, through a Congregation for the Propagation of the Faith decree, in 1882, abolished the phrase Latin: in partibus infidelium and ordered that future appointments should be made as "titular bishops". The custom, when Boudinhon wrote his article, was to join to the name of the see that of the district to which it formerly belonged, or else merely to say "titular bishop".
The Annuaire Pontifical Catholique published a very complete list of the titular sees and titular bishops. Although it did not claim to be perfect, it contained the names of the sees and the bishops who had held the titles as far back, in some cases, as the 14th century.
Titular sees, according to Corrigan in 1920, were conferred on
In the context of improved relations with the Eastern Orthodox Church and Oriental Orthodoxy after the Second Vatican Council, the Holy See, while continuing to appoint bishops to titular sees in North Africa, ceased to make such appointments to sees that were historically part of the Eastern patriarchates of Constantinople, Alexandria, Antioch, and Jerusalem. It began instead to treat as titular sees also those Catholic dioceses in any country no longer used as titles of diocesan bishops because of having been absorbed into other dioceses or having been renamed due to a change of the bishop's place of residence. The change of practice is reflected in the inclusion from then on of such sees in the official lists of titular sees in editions of the Annuario Pontificio.
Previously, titular sees were assigned not only to auxiliary bishops but also to retired bishops and coadjutor bishops. That practice was replaced by the present one of referring to a retired bishop as a bishop emeritus of the see that he held, and to a coadjutor bishop simply as coadjutor bishop of the see to which he has been appointed. This change too is reflected in editions of the Annuario Pontificio of the period, which include information on renunciation by retired and coadjutor bishops of titular sees to which they had been appointed.
The crusading William IV, Count of Nevers, dying in the Holy Land in 1168, left the building known as the Hospital of Panthenor in the town of Clamecy in Burgundy, together with some land, to the Bishops of Bethlehem, in case Bethlehem should fall under Muslim control. After Saladin took Bethlehem in 1187, the Bishop took up residence in 1223 in his property, which remained the seat of titular Bishops of Bethlehem for almost 600 years, until the French Revolution of 1789. 
When Bishop Jacques Gaillot of the residential Diocese of Évreux, controversial for his positions on religious, political and social matters, refused in 1995 to retire (and thus become Bishop Emeritus of Évreux), he was transferred to the titular see of Partenia.
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 residing in Oxford, England, is titled Bishop of Diokleia; the bishop of the Russian Orthodox Diocese of Sourozh is the Russian Orthodox Church's bishop in the United Kingdom.