A superluminal radio source is a radio source in which internal motions (for example, the increasing separation between the core and a knot of material in a jet) appear faster than the speed of light in our frame of reference. The data are consistent with this being a transformation effect from seeing jets moving almost directly toward us, so that the emitting material almost catches up with its own radiation. This has the effect of compressing the scale of time that we measure for it, and so increasing the observed speed.