A.P.S.E

In architecture, an apse is a semicircular recess covered with a hemispherical vault or semi-dome, also known as an exedra. In Byzantine, Romanesque, and Gothic Christian church architecture, the term is applied to a semi-circular or polygonal termination of the main building at the liturgical east end, regardless of the shape of the roof, which may be flat, sloping, domed, or hemispherical. Small... (Wikipedia)