The Collaborative International Dictionary of English v.0.48:

Radius vector \Ra"di*us vec"tor\ (v[e^]k"t[o^]r).
   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 coordinates. See Coordinate, n.
      [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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