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."