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.n. In ordinary use, the termination of the choir orn. 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.n. . In astronomy, same as apsis.