What does apsis mean?
The definition of apsis
Ap”sis, n.; pl. Apsides. See Apse.
Etym: [L. apsis, absis, Gr.

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.
3 (Arch.)
Same as Apse.