The apparent reversal in the motion of a planet across the sky relative to the background stars, caused by the Earth passing it or being passed by it.
Figure: GIF animation showing Mars against the background stars at the same time on successive nights from July 1, 1988 to January 1, 1989. Mars moves in one direction, then backs up for a time (retrograde motion) before moving forward again. The other object that pass through the image from time to time is the moon.