Source : Webster's Revised Unabridged Dictionary (1913)

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.

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

   3. (Arch.) Same as {Apse}.

Source : WordNet®

apsis
     n : a domed or vaulted recess or projection on a building
         especially the east end of a church; usually contains the
         altar [syn: {apse}]
     [also: {apsides} (pl)]