noun ·same·as apse.
2. apsis ·noun in a curve referred to polar coordinates, any point for which the radius vector is a maximum or minimum.
3. apsis ·noun 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.