radius vector

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A line segment that joins the origin and a variable point in a system of polar or spherical coordinates.
• noun The length of such a line segment.
• noun Astronomy A straight line connecting the center of mass of a satellite, such as a planet, moon, or comet, to the center of mass of the body it orbits, such as the sun or Earth.

from the GNU version of the Collaborative International Dictionary of English.

• (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 coördinates. See coördinate, n.
• (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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun mathematics 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.
• noun astronomy 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.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a line connecting a satellite to the center of the body around which it is rotating
• noun a line connecting a point in space to the origin of a polar coordinate system