Apse

In architecture, an apse (plural apses; from Latin absis 'arch, vault' from Ancient Greek ἀψίς apsis 'arch'; sometimes written apsis, plural apsides) is a semicircular recess covered with a hemispherical vault or semi-dome, also known as an exedra. In Byzantine, Romanesque, and Gothic Christian church (including cathedral and abbey) architecture, the term is applied to a semi-circular or polygonal termination of the main building at the liturgical east end (where the altar is), regardless of the shape of the roof, which may be flat, sloping, domed, or hemispherical. Smaller apses are found elsewhere, especially in shrines.[1]

Typical early Christian Byzantine apse with a hemispherical semi-dome in the Basilica of Sant'Apollinare in Classe
Typical floor plan of a cathedral, with the apse shaded