apsis

noun, plural ap·si·des [ap-si-deez] /ˈæp sɪˌdiz/.

Astronomy. either of two points in an eccentric orbit, one (higher apsis) farthest from the center of attraction, the other (lower apsis) nearest to the center of attraction.
Architecture. an apse.

noun plural apsides (æpˈsaɪdiːz, ˈæpsɪˌdiːz)

Also called: apse either of two points lying at the extremities of an eccentric orbit of a planet, satellite, etc, such as the aphelion and perihelion of a planet or the apogee and perigee of the moon. The line of apsides connects two such points and is the principal axis of the orbit
another name for apse (def. 1)

“perigree of the moon, perihelion of a planet” (plural apsides), 1650s, from Latin apsis “arch, vault” (see apse).

Plural apsides (ăp′sĭ-dēz′)

In the path of an orbiting body, either of the two points at which it is closest to or farthest away from the body it is orbiting. See also apoapsis periapsis.