Hagia Sophia, officially the Hagia Sophia Grand Mosque, is a mosque, a former church, and a major cultural and historical site in Istanbul, Turkey. The last of three church buildings to be successively erected on the site by the Eastern Roman Empire, it was completed in 537 AD. The site was an Chalcedonian church from 360 AD to 1054, an Orthodox church following the Great Schism of 1054, and a Catholic church following the Fourth Crusade. It was reclaimed in 1261 and remained Eastern Orthodox until the fall of Constantinople in 1453. It served as a mosque until 1935, when it became a museum. In 2020, the site once again became a mosque.

A cathedral is a church that contains the cathedra of a bishop, thus serving as the central church of a diocese, conference, or episcopate. Churches with the function of "cathedral" are usually specific to those Christian denominations with an episcopal hierarchy, such as the Catholic, Eastern Orthodox, Anglican, and some Lutheran churches. Church buildings embodying the functions of a cathedral first appeared in Italy, Gaul, Spain, and North Africa in the 4th century, but cathedrals did not become universal within the Western Catholic Church until the 12th century, by which time they had developed architectural forms, institutional structures, and legal identities distinct from parish churches, monastic churches, and episcopal residences. The cathedral is more important in the hierarchy than the church because it is from the cathedral that the bishop governs the area under his or her administrative authority.