A domed or vaulted semicircular or polygonal recess, especially at the east end of a church (4)
I believe the answer is:
apse
(Other definitions for apse that I've seen before include "room for an altar" , "Peas (anag) -- part of a church" , "Where the altar is usually found in a church" , "Recess at eastern end of a church" , "Church area" .)