nSemidome(Arch) A roof or ceiling covering a semicircular room or recess, or one of nearly that shape, as the apse of a church, a niche, or the like. It is approximately the quarter of a hollow sphere.
Century Dictionary and Cyclopedia
nsemidomeHalf a dome, especially as formed by a vertical section; less properly, any feature of form or construction more or less similar to half a dome. The term applies especially to such quadrantal vaults as those which cover in the apse of most Italian medieval churches, and of many French and German Romanesque churches. See also cut under apse.
In the semidome of the apse is Christ crowning the Madonna, with angels, sibyls and prophets.
"Encyclopaedia Britannica, 11th Edition, Volume 16, Slice 6" by Various