Part of the Hagia Sophia with a celebrated mosaic of Mary and child (4)
I believe the answer is:
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(Other definitions for apse that I've seen before include "Where the altar is usually found in a church" , "Primate holds saint to be a" , "Where you could find a magistrate's chair" , "Part of a church with the altar" , "Projection to the east end of a church" .)