Semi-dome

A semi-dome(also called a "half-dome") is the term in architecture for half a dome. It is used to cover a semi-circular area, usually an apse. A semi-dome was often used at each end of a barrel vaulted ceiling. 
History.
An early use was to cover Roman cult images. They are a feature found in Ancient Roman and traditional church architecture. The semi-dome is featured in Byzantine architecture and Early Christian architecture. They are prominent in mosques and iwans in Islamic architecture. Some mosques featured semi-domes decorated with glass mosaics. In the second half of the 6th century Ottoman architecture began using half-domes combined with domes. Semi-domes were common in Roman baths, palaces and large buildings. During the Dark Ages in Britain whenever an apse was built, a semi-dome covered it. In 11th century Normandy every apse of every church had a semi-dome over it. The semi-dome went out of use when the east end of churches were squared.