An Apse

1. (architecture) A semicircular projection from a building, especially the rounded east end of a church that contains the altar.
2. The bishop's seat or throne in ancient churches.
3. A reliquary, or case in which the relics of saints were kept.
4. (astronomy, obsolete) The nearest and furthest points to the centre of gravitational attraction for a body in orbit. More usually called an apsis.