apoapsis and periapsis

An apsis is either of the points in the orbit of a celestial object that is farthest from or closest to the body being orbited. The closest point is the periapsis, the further point the apoapsis. The line joining these points is called the line of apsides, and is another name for the major axis of the orbit. Rotation of the line of apsides in the plane of the orbit is known as apsidal motion or advance of perihelion (in the case of a planet orbiting the Sun) or advance of pericenter (in the case of a binary star). Plural is "apsides."