Apsis /(ăp"sĭs)/

pl. Apsides ((ăp"sĭ*dēz))

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 other, the lower apsis; and the line joining them, the line of apsides. (Astron.)
2. In a curve referred to polar coördinates, any point for which the radius vector is a maximum or minimum. (Math.)
3. Same as Apse. (Arch.)