In architecture, apsis is a synonym for apse.

Aphelia redirects here. For the moth genus, see Aphelia (moth).

Apogee redirects here. For the video game publishing and developing company, see Apogee Software. For the album by Bongzilla, see Apogee (album).

In astronomy, an apsis, plural apsides (IPA: //) is the point of greatest or least distance of the elliptical orbit of an astronomical object from its center of attraction, which is generally the center of mass of the system.

The point of closest approach is called the periapsis or pericentre and the point of farthest excursion is called the apoapsis (Greek από, from, which becomes απ before a vowel, and αφ before rough breathing), apocentre or apapsis (the latter term, although etymologically more correct, is much less used). A straight line drawn through the periapsis and apoapsis is the line of apsides. This is the major axis of the ellipse, the line through the longest part of the ellipse.