This word is also written apsis and absis.
Ap"si*dal (p"s*dal), a. 1. (Astron.) Of or pertaining to the apsides of an orbit.
2. (Arch.) Of or pertaining to the apse of a church; as, the apsidal termination of the chancel.
Ap"si*des (p"s*dz), n. pl. See Apsis.
||Ap"sis (p"ss), n.; pl. Apsides (p"s*dz). 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.
2. (Math.) In a curve referred to polar coördinates, any point for which the radius vector is a maximum or minimum.