Apsis

• POS: Noun
• HYPHEN: ap=sis

CIDE DICTIONARY

Apsis, n. [L. apsis, absis, Gr. "apsi`s, "apsi^dos, a tying, fastening, the hoop of a wheel, the wheel, a bow, arch, vault, fr. "a`ptein to fasten.].

•  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.  [1913 Webster]
•  In a curve referred to polar coördinates, any point for which the radius vector is a maximum or minimum.  [1913 Webster]
•  Same as Apse.  [1913 Webster]

OXFORD DICTIONARY

Apsis, n. (pl. apsides) either of two points on the orbit of a planet or satellite that are nearest to or furthest from the body round which it moves.

Derivative

apsidal adj.

Etymology

L f. Gk (h)apsis, -idos arch, vault