Apsis
AP'SIS, noun plu apsides. [Gr. connection, from to connect.]
1. In astronomy, the apsides are the two points of a planet's orbit, which are at the greatest and least distance from the sun or earth; the most distant point is the aphelion, or apogee; the least distant, the perihelion or perigee. The line connecting these is called the line of the apsides.
2. apsis or absis is the arched roof of a house, room or oven; also the ring or compass of a wheel.
3. In ecclesiastical writers, an inner part of a church, where the altar was placed, and where the clergy sat, answering to the choir and standing opposite to the nave. Also, the bishop's seat or throne in ancient churches; called also exedra and tribune. This same name was given to a reliquary or case in which the relics of saints were kept.