Apse or Apsis

(ἀψίς, Lat. absis, prob. for ἃψις, a juncture or vaulted arch), is a term used by ecclesiastical writers to designate

1. that part of the interior of ancient churches where the bishop and clergy had their seats. The form of the apsis was hemispherical, and it consisted of two parts: one, the choir or presbytery; the other, the sanctuary. The choir always terminated toward the east in a semicircle, round which were the seats of the clergy, having in the middle the throne of the bishop or superior, which was raised above the others. The term came into use in the 8th century to denote the deepest recess behind the altar in the Eastern Churches.

2. It was also commonly used for the bishop's throne, called apsis gradata, being raised by means of steps.

3. The word at other times denotes the case in which the relics of saints were kept, which was round or arched at the top, and commonly placed on the altar: it was usually of wood, sometimes also of gold and silver, and occasionally beautifully sculptured.

4. In later church architecture, it is used to denote any semicircular or polygonal termination of the choir, or other portion of a church. — Bingham, Orig. Eccl. bk. 8, ch. 3; Lenoir, Architect. Monast. (Paris, 1852).