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.

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".

frigidarium
A large cold pool to drop into after enjoying a hot Roman bath (from frigeo). Normally frigidarium has used after a visit to warm rooms (caldarium) or after a training in palaistra. As the largest room in the thermae and often functioned as a hall for social events or communication

hydraulic mortar
A waterproofing mortar, applied to tha walls of spaces exposed to water and humidity, such as cisterns, baths etc.

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.

ring-type baths
Architectural type of bath complexes, in which the arrangement from the frigidarium (cold chamber) to the other halls imposes a circulation ring in one direction only.