The Collaborative International Dictionary of English v.0.48:

Apse \Apse\ ([a^]ps), n.; pl. Apses ([a^]p"s[e^]z). [See
   Apsis.]
   1. (Arch.)
      (a) 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. Hence:
      (b) The bishop's seat or throne, in ancient churches.
          [1913 Webster]

   2. A reliquary, or case in which the relics of saints were
      kept.
      [1913 Webster]

   Note: This word is also written apsis and absis.
         [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Apsis \Ap"sis\ ([a^]p"s[i^]s), n.; pl. Apsides
   ([a^]p"s[i^]*d[=e]z). See Apse. [L. apsis, absis, Gr.
   "apsi`s, "apsi^dos, a tying, fastening, the hoop of a wheel,
   the wheel, a bow, arch, vault, fr. "a`ptein to fasten.]
   1. (Astron.) One of the two points of an orbit, as of a
      planet or satellite, which are at the greatest and least
      distance from the central body, corresponding to the
      aphelion and perihelion of a planet, or to the apogee and
      perigee of the moon. The more distant is called the higher
      apsis; the other, the lower apsis; and the line joining
      them, the line of apsides.
      [1913 Webster]

   2. (Math.) In a curve referred to polar co["o]rdinates, any
      point for which the radius vector is a maximum or minimum.
      [1913 Webster]

   3. (Arch.) Same as Apse.
      [1913 Webster]

WordNet (r) 3.0 (2006):

apsis
    n 1: a domed or vaulted recess or projection on a building
         especially the east end of a church; usually contains the
         altar [syn: apse, apsis]