from
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.
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2. (Math.) In a curve referred to polar co["o]rdinates, any
point for which the radius vector is a maximum or minimum.
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3. (Arch.) Same as {Apse}.
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