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Origin: L. Apsis, absis, gr, a tying, fastening, the hoop of a wheel, the wheel, a bow, arch, vault, fr. To fasten.

1. (Science: astronomy) 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. (Science: mathematics) in a curve referred to polar coordinates, any point for which the radius vector is a maximum or minimum.

3. Same as Apse.