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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.

arch, the
A curved structure, as a masonry, that covers openings in the stonework and is capable to supports the weight of material over an open space, as in a bridge, doorway, etc. It is often used as a decorative element.

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

caldarium
Derivative of the Latin verb caleo (= warm up). It is the strongly heated room of Roman baths. Its hot plunge pool was used to take not only a hot bath but also a steam bath due to high levels of humidity. It was also called the "inner room".

hypocaust, the
the main system for the heating of ancient baths. The word means literally a “furnace that burns underneath”. With this system the room’s floor was supported by small poles and the space underneath the floor was heated by the circulation of hot air, while the heat was transferred through the walls by conductors.

niche
Semi-circular recess on the surface of the wall.