A short-lived, high amplitude discharge of electricity that accompanies a cloud-to-ground lightning event. The bright lightning flash that is visible to the human eye is actually composed of extremely rapid electric discharges called strokes. The return stoke follows a downward extending leader, or conductive path of ionized air. The electrons in the cloud flow down this conductive pathway toward the surface of the earth. As electrons continually migrate down the path, electrons remaining higher up on the path in the cloud begin to consecutively move down the channel to the surface. Since the path of electron flow is progressively lengthened upward, the discharge of the elevated electrons high in the cloud to a lower place in the cloud and then down the pathway to the ground is called the return stroke.
[Journal of Atmospheric and Solar-Terrestrial Physics, v. 62, Issue 3, p. 169-187]
Source: Atmospheric Chemistry Glossary