radial velocity

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

• noun The velocity, or a component of the velocity, of an object as it moves toward or away from an observer, as a star or galaxy observed from Earth. Periodic changes in the radial velocities of stars may indicate the presence of orbiting planets even if the planets themselves are not directly observable.

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

• noun astronomy motion of a star toward or away from the earth.

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

• noun velocity along the line of sight toward or away from the observer