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An arched srtucture or a semi-circular end of a wall. In byzantine architecture it means the semicircular, usually barrel-vaulted, niche at the east end of a basilica. The side aisles of a basilica may also end in an apse, but it is always in the central apse where the altar is placed. It was separated from the main church by a barrier, the templon, or the iconostasis. Its ground plan on the external side could be semicircular, rectangular or polygonal.

dome
A characteristic element of Byzantine architecture. The dome is a hemispherical vault on a circular wall (drum) usually pierced by windows. The domed church emerges in the Early Byzantine years and its various types gradually prevail, while they are expanded in the Balkans and in Russia.

narthex
A portico or a rectangular entrance-hall, parallel with the west end of an early Christian basilica or church.

transitional domed basilica
A domed basilica, like the Justinianic churches; however, in these transitional basilicas, several openings are piercing the piers that carry the dome. Thus the massive piers begin to gradually dissolve into pillars and finally columns, while they leave space to the corner bays; from these alterations eventually emerges the cross-in-square type.