Apsis
Pronunciation : Ap"sis
Part of Speech : n.; pl. Apsides. See Apse.
Etymology : [L. apsis, absis, Gr.
Definition : 1. (Astron.)
Defn: 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.)
Defn: In a curve referred to polar co?rdinates, any point for which the radius vector is a maximum or minimum.
3. (Arch.)
Defn: Same as Apse.
Source : Webster's Unabridged Dictionary, 1913