\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}.