Definitions for "Radial velocity"
the speed at which an object is moving away or toward an observer. By observing spectral lines, astronomers can determine how fast objects are moving away from or toward us; however, these spectral lines cannot be used to measure how fast the objects are moving across the sky. (See also text in Spec and Mov.)
the rate of change of the distance to an object.
speed toward or away from the observer.