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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.

barrel-vault
vaulted, semi-cylindrical construction used often as roof.

bema
The area at east end of the naos in Byzantine churches, containing the altar, also referred to as the presbetery or hierateion (sanctuary). In these area take place the Holy Eucharist.

Horn of Amaltheia (cornucopia)
(also known as the horn of plenty) In Greek mythology, Amaltheia was the goat who breast-fed the child Zeus. When he accidentally broke one of her horns while playing, he turned it into a golden one, which would give to the holder whatever that he wished for. In antiquity, the Horn of Amaltheia was a symbol of plenty and prosperity. It was adopted by the Christians endowed with connotations of the Providence, hope and compassion.

iconographic programme
The elaboration and arrangement of wall paintings in a Byzantine church, according to unwritten rules. For example, the customary depiction of Christ Pantokrator on the dome.

interlaced circles
Decorative motif of successive adjoining circles, sometimes adjoined to each other by knots. It was often used to form a frame, while each circle may frame a bust, a scene or other motifs.

intrados, soffit
The internal surface of an arch or barrel-vault.

Maltese cross
A cross with four equal "V"-shaped arms, joined together at their tips; that way each arm has two points.

non figural (anti-iconic) decoration
Decoration which consists only of geometrical and floral motifs. This kind of decoration was widely spread in the tombs of the Early Byzantine period and during Iconoclasm. In this case it is reffered to as anti-iconic, since the iconoclasts rejected icon veneration and prohibited any depiction of the human figure of Christ, Virgin Mary, the saints and angles.

triumphal arch
(Rom.:) A structure in the shape of a monumental archway, built to celebrate the victory of a Roman general or Emperor.(Byz. Archit.) The arch formed above the Horaia Pyle (Royal Door), which frames the curve of the conch of the apse and separates the bema from the nave.