An apsis (Greek: ἁψίς; plural apsidesz/, Greek: ἁψῖδες) is an extreme point in the orbit of an object. The word comes via Latin from Greek and is cognate with apse. For elliptic orbits about a larger body, there are two apsides, named with the prefixes peri- (from περί (peri), meaning 'near') and ap-/apo- (from ἀπ(ό) (ap(ó)), meaning 'away from') added to a reference to the body being orbited.
For a body orbiting the Sun, the point of least distance is the perihelion (n/), and the point of greatest distance is the aphelion (n/).
The terms become periastron and apastron when discussing orbits around other stars.
For any satellite of Earth, including the Moon, the point of least distance is the perigee (iː/) and greatest distance the apogee.
For objects in lunar orbit, the point of least distance is sometimes called the pericynthion (n/) and the greatest distance the apocynthion (n/). Perilune and apolune are also used.
For an orbit around any barycenter, the terms periapsis and apoapsis (or apapsis) are used. Pericenter and apocenter are equivalent alternatives.
A straight line connecting the periapsis and apoapsis is the line of apsides. This is the major axis of the ellipse, its greatest diameter. The center of mass, or barycenter, of a two-body system lies on this line at one of the two foci of the ellipse. When one body is sufficiently larger than the other, this focus may be located within the larger body. However, whether this is the case, both bodies are in similar elliptical orbits. Both orbits share a common focus at the system's barycenter, with their respective lines of apsides being of length inversely proportional to their masses.
Historically, in geocentric systems, apsides were measured from the center of the Earth. However, in the case of the Moon, the barycenter of the Earth–Moon system (or the Earth–Moon barycenter) as the common focus of both bodies' orbits about each other, is about 75% of the way from Earth's center to its surface.
In orbital mechanics, the apsis technically refers to the distance measured between the barycenters of the central body and orbiting body. However, in the case of spacecraft, the family of terms are commonly used to refer to the orbital altitude of the spacecraft from the surface of the central body (assuming a constant, standard reference radius).