Originally, double-tracking meant the recording of a vocal track on one tape recorder track, then listening to this while recording another similar track. The two tracks are combined and re-recorded into a single track, which will sound more diffuse due to slight differences in the two original tracks as double-tracking produces a slight chorus effect to voices. In this case, it is also called re-tracking. Double-tracking can be done with DSP which introduce a small randomly varying time delays to one signal and then combine it with the original signal.
See delay(3), stereoizing.