In a curve referred to polar coördinates, any point for which the radius vector is a maximum or minimum
{i} points of an orbit that are furthest and nearest the center (Astronomy); crescent-shaped channel
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