An apsis (plural apsides /ˈæpsɪdz/ AP-sih-deez, from Greek "orbit") is the farthest or nearest point in the orbit of a planetary body about its primary body. The apsides of Earth's orbit of the Sun are two: the aphelion, where Earth is farthest from the sun, and the perihelion, where it is nearest. "Apsides" can also refer to the distance of the extreme range of an object orbiting a host body.

The apsides refer to the farthest (1) and nearest (2) points reached by an orbiting planetary body (1 and 2) with respect to a primary, or host, body (3).
*The line of apsides is the line connecting positions 1 and 2.
*The table names the (two) apsides of a planetary body (X, "orbiter") orbiting the host body indicated:
(1) farthest(X) orbiter(3) host(2) nearest
aphelionHalley's CometSunperihelion
apocentercomet, e.g.primarypericenter
apoapsiscomet, e.g.primaryperiapsis
For example, the Moon's two apsides are the farthest point, apogee, and the nearest point, perigee, of its orbit around the host Earth. The Earth's two apsides are the farthest point, aphelion, and the nearest point, perihelion, of its orbit around the host Sun. The terms aphelion and perihelion apply in the same way to the orbits of Jupiter and the other planets, the comets, and the asteroids of the Solar System.
The two-body system of interacting elliptic orbits: The smaller, satellite body (blue) orbits the primary body (yellow); both are in elliptic orbits around their common center of mass (or barycenter), (red +).
∗Periapsis and apoapsis as distances: The smallest and largest distances between the orbiter and its host body.
Keplerian orbital elements: point F, the nearest point of approach of an orbiting body, is the pericenter (also periapsis) of an orbit; point H, the farthest point of the orbiting body, is the apocenter (also apoapsis) of the orbit; and the red line between them is the line of apsides.