A straight line (or the length of such line) connecting any point, as of a curve, with a fixed point, or pole, round which the straight line turns, and to which it serves to refer the successive points of a curve, in a system of polar coördinates. See Coördinate, n.

An ideal straight line joining the center of an attracting body with that of a body describing an orbit around it, as a line joining the sun and a planet or comet, or a planet and its satellite.

a line connecting a satellite to the center of the body around which it is rotating

a line connecting a point in space to the origin of a polar coordinate system

a line pointing from the center of the circle to the point on the rim of the circle)