Apse
[aps]
Etymology: Medieval Latin & Latin; Medieval Latin apsis, from Latin

1) The domed or vaulted east end of the church. In Britain the apse is generally squared off, while on the continent, rounded apses were common. (Glossary of Church Architecture, retrieved from BritainExpress.com)

2) Part of a building semi-circular in plan. (Martindale, Andrew. Gothic Art, 268)

3) Semicircular or polygonal end to a building. (Wood, Margaret. The English Medieval House, 409)

*Definitions #2 and #3 retrieved from Netserf’s Medieval Glossary (http://www.netserf.org/Glossary)