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American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Architecture A usually semicircular or polygonal, often vaulted recess, especially the termination of the sanctuary end of a church.
2. n. Astronomy An apsis.

Century Dictionary and Cyclopedia

1. n. . In architecture: Strictly, any recess, or the termination of a building, of semicircular plan, covered by a semicircular vault or semi-dome; hence, a similar feature of polygonal plan.
2. n. In ordinary use, the termination of the choir or
3. n. sanctuary of any church, particularly if it presents a superficial resemblance to an apse in the stricter sense, in that it is at least approximately semicircular in plan, and vaulted: commonly equivalent to chevet, and applied to the altar extremity of a church, even if of rectangular plan and not vaulted, and including the apse-aisles, chapels, and any other adjunct to the ritual east end of a church. The apse in its origin was a characteristic feature of the ancient Roman basilica, in which it formed the raised tribune for the court magistrates. The throne of the quæstor or presiding judge stood in the center of the chord of the are of the apse. When the basilicas became Christian churches, the throne was replaced by the high altar, which still occupies this position in Latin churches of the strict basilica type, and has regularly kept it in Oriental churches. Some types of church regularly have secondary apses in other positions than at the eastern end, as at the western end, at the extremities of the transepts or of aisles, etc. See cuts under basilica and bema. Also apsis.
4. n. . In astronomy, same as apsis.

Wiktionary

1. n. A semicircular projection from a building, especially the rounded east end of a church that contains the altar.
2. n. The nearest and furthest points to the centre of gravitational attraction for a body in orbit. More usually called an apsis.
3. n. In some local dialects and in archaic usage, an aspen tree.

GNU Webster's 1913

1. n. A projecting part of a building, esp. of a church, having in the plan a polygonal or semicircular termination, and, most often, projecting from the east end. In early churches the Eastern apse was occupied by seats for the bishop and clergy.
2. n. The bishop's seat or throne, in ancient churches.
3. n. A reliquary, or case in which the relics of saints were kept.

WordNet 3.0

1. n. a domed or vaulted recess or projection on a building especially the east end of a church; usually contains the altar