Apsis
Apsis (ap"sis) , noun
[Latin apsis, absis, Greek "apsi`s, "apsi^dos, a tying, fastening, the hoop of a wheel, the wheel, a bow, arch, vault, from "a`ptein to fasten.]
1.
(Astronomy) 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.
(Mathematics) In a curve referred to polar coordinates, any point for which the radius vector is a maximum or minimum.
3.
(Architecture) Same as Apse.