Apse of a Cathedral
An illustration depicting the inside of an apse. In architecture, the apse (Latin absis "arch, vault"; sometimes written apsis; plural apses) is a semicircular recess covered with a hemispherical vault. In Romanesque, Byzantine and Gothic Christian abbey, cathedral and church architecture, the term is applied to the semi-circular or polygonal section of the sanctuary at the liturgical east end beyond the altar. Geometrically speaking, an apse is either a half-cone or half-dome.