apsis

‖n.; pl. Apsides 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. [1913 Webster]

2. (Math.) In a curve referred to polar coördinates, any point for which the radius vector is a maximum or minimum. [1913 Webster]

3. (Arch.) Same as Apse. [1913 Webster]