Origin: L. Apsis, absis, Gr, a tying, fastening, the hoop of a wheel, the wheel, a bow, arch, vault, fr. To fasten.
1. 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 another, the lower apsis; and the line joining them, the line of apsides.
2. In a curve referred to polar coordinates, any point for which the radius vector is a maximum or minimum.
3. Same as Apse.
Source: Websters Vocabulary