ambulatory (byz. arch.)
A continuous passage that envelops the naos or the centrally planned core of a structure. In a cross-domed church, where the dome is supported on four masonry piers and between each pair of piers two columns are inserted, the ambulatory is formed by the lateral aisles and western part of the church. Later on, an ambulatory could also envelop a cross-in-square core. During the Palaeologan period, ambulatories, usually serving as funerary chambers, were added to many middle-Byzantine churches of Constantinople.

apse
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.

basilica
In ancient Roman architecture a large oblong type building used as hall of justice and public meeting place. The roman basilica served as a model for early Christian churches.

cross-in-square church
Type of church in which four barrel-vaulted bays form a greek cross; the central square of their intersection is domed. The cross is inscribed into the square ground plan by means of four corner bays.

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