(1):
(n.) One of the two points of an orbit, as of a planet or satellite, which are at the greatest and least distance from the central body, corresponding to the aphelion and perihelion of a planet, or to the apogee and perigee of the moon. The more distant is called the higher apsis; the other, the lower apsis; and the line joining them, the line of apsides.
(2):
(n.) Same as Apse.
(3):
(n.) In a curve referred to polar coordinates, any point for which the radius vector is a maximum or minimum.