Source : Webster's Revised Unabridged Dictionary (1913)

Radius vector \Ra"di*us vec"tor\
   1. (Math.) 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 co["o]rdinates. See {Co["o]rdinate}, n.

   2. (Astron.) 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.

Source : WordNet®

radius vector
     n 1: a line connecting a satellite to the center of the body
          around which it is rotating
     2: a line connecting a point in space to the origin of a polar
        coordinate system