The Collaborative International Dictionary of English v.0.48:

Apsides \Ap"si*des\ ([a^]p"s[i^]*d[=e]z), n. pl.
   See Apsis.
   [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]