Apsis

In astronomy, an apsis (plural apsides) is either of the two points in an orbit at which the orbiting body makes its closest or farthest approach to the primary.

The point of closest approach is called the periapsis (from the Greek peri around or, as in this case, next to).

The point of farthest approach is called the apoapsis or apapsis (from the Greek apo away from).

Terminology
Astronomers often change the root word apsis to a root specific to the primary under discussion. Most of these roots are Greek-derived, although some Latin-derived roots have occasionally appeared in the technical literature or even on the Internet.

The following table lists the proper roots, including some Latin-derived roots:

Related Links
Apsis by Wikipedia

Apside Apside