A line connecting a point in space to the origin of a polar coordinate system WordNet 3.0 & Open English WordNet

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 coordinates. Wiktionary

A straight line (or the length of such line) connecting any point. Wiktionary (translation)