semidome

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A roof covering a semicircular space; half a dome.

from The Century Dictionary.

• noun Half 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.

from the GNU version of the Collaborative International Dictionary of English.

• noun (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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun architecture Half of a dome ("cut" vertically), used to cover a semicircular area.