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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.

arcosolium
(lat.) A tomb carved out of a wall (solium) with an arched niche (arcus) above it.

mausoleum
A type of large-scale burial monument, named after the tomb of Mausolus, satrap of Caria.

opus mixtum (ουδ.)
Masonry consisting of small stones and abudant mortar, which often interchange with horizontal double rows of plinths.

opus tessellatum, the
The mosaic art for Romans. Its domination spans a period from Hellenistic till Byzantine times. These mosaics consist of small and equal in size stone, clay or glass colourful tesserae.