Apse
From Churches of Rome Wiki
An apse is a semi-circular or polygonal recess at the end of a building, either projecting from it or subsituting one of the walls. In churches it is normally part of the sanctuary.
In Rome, apses are traditionally richly decorated with mosaics or paintings.
In Eastern churches, it is common with a triple apse, and this feature has been preserved in some Roman churches built or designed by Greeks.
