n. A semicircular recess, especially at the end of the sanctuary in a church. Often topped with a semicircular dome.
Dictionnaire raisonné de l’architecture française du XIe au XVIe siècle
Eugène Viollet-le-duc (1814-1879) -- a preservationist of gothic castles and cathedrals-- is considered by some to have been the first modern architectural theorist, with heavy influence on the future of modernism.
The etching images posted here were published by Ernst Gründ (Paris), c1854. The full digital document of an 1875 edition can be found on archive.org, courtesy of The Getty Center Library.