What is the meaning of Apsis?

A recess or projection, with a dome or vault, at the east end of a church; an apse.

Either of the points in the elliptical orbit of a planet, comet, or moon where it is closest or furthest from the body it orbits; examples are perihelion, aphelion, perigee, and apogee

An apse (architectural element).

arch, vault

Source: wiktionary.org