ap·sis / ˈapsis/ • n. (pl. -si·des / -siˌdēz/ ) either of two points on the orbit of a planet or satellite that are nearest to or furthest from the body around which it moves.DERIVATIVES: ap·si·dal / ˈapsidl/ adj.

apsis (pl. apsides) Either of two points in an object's orbit. The closest point to the primary body is known as the periapsis, and the furthest the apapsis. The apsides of the Earth's orbit are its perihelion and aphelion; in the Moon's orbit they are its perigee and apogee. The line of apsides connect these points.