An apsis, plural apsides, is the point of greatest or least distance of a body from one of the foci of its elliptical orbit. In modern celestial mechanics this focus is also the center of attraction, which is usually the center of mass of the system. Historically, in geocentric systems, apsides were measured from the center of the Earth.
OmegaWiki Dictionary
Ω
apsis Point of greatest or least distance of the elliptical orbit of an astronomical object from the center of mass of the system, e. g. perihelion or aphelion