APSE

The Apse is a semicircular or polygonal termination to the choir or aisles of a church. It is that part of the church where the clergy are seated or the altar placed. It was so called from being usually domed or vaulted, and was so used by the Greeks and Romans. The term is sometimes applied to a canopy over an altar; a dome; the arched roof of a room; the bishop's seat in old churches; a reliquary; a recess, semicircular in plan, covered over with a vault in the shape of a semi-dome or any other description of roof. The apse is always solid below though generally broken by windows above.

HAGIA SOPHIA APSE
Picture taken from : http://www.sacred-destinations.com/turkey/istanbul-hagia-sophia-photos/slides/apse-c-hbetts

Acknowledgment: http://www.newadvent.org/cathen/01659a.htm