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After the Roman Empire became officially Christian (Edict of Thessalonica), the term came by extension to refer to a large and important church that has been given special ceremonial rites by the Pope. Thus the word retains two senses today, one architectural and the other ecclesiastical.
• Cathedral
A cathedral is a church, usually Roman Catholic, Anglican, Oriental Orthodox or Eastern Orthodox, housing the seat of a bishop. The word cathedral takes its name from the word cathedra, or Bishop's Throne (In Latin: ecclesia cathedralis). The term is sometimes (improperly) used to refer to any church of great size.
The church that has the function of cathedral is not of necessity a large building. It might be as small as Christ Church Cathedral in Oxford, England, Sacred Heart Cathedral in Raleigh, United States, or Chur Cathedral in Switzerland. But frequently, the cathedral, along with some of the abbey churches, was the largest building in any region.