Apse

In architecture, the apse is a semicircular recess covered with a hemispherical vault or semi-dome.

In Romanesque, Byzantine and Gothic Christian abbey, cathedral  and church 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.

The epithet "apsidal" may be applied to the exedra of classical architecture, a feature of the secular Roman basilica, which provided the initial prototype for Early Christian churches. The apse in the Roman basilica was often raised (as the sanctuary generally still is) as a hieratic feature, the "tribuna", that set apart the magistrates who deliberated within it. Where an apse is raised by steps, especially if it contains a throne, it can be architecturally referred to as a tribune, though this term is rarely used in discussing churches.

The apse as a semicircular projection may be roofed with a semi-dome (also called a half-dome) or with radiating vaulting. A simple apse may be merely embedded within the wall of the east end. Eastern orthodox churches may have a triple apse, which is usually a mark of Byzantine influence when it is seen in Western churches.

Smaller subsidiary apses may be found around the choir or at the ends of transepts. These proliferating apses are common in later Byzantine architecture and the Ottoman architecture that developed from it. An exedra or apse may be reduced in scale to form a niche within the thickness of walling; a niche does not reveal its presence by projecting on the exterior.

The interior of the apse, especially the semi-dome, is traditionally a focus of iconography, bearing the richest concentration of mosaics, or painting and sculpture, towards which all other decoration may tend.