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 coÃ¶rdinates, any point for which the radius vector is a maximum or minimum.

3 (Arch.)

Same as Apse.