The shadow period is an astrological term describing the period of time that it takes a retrograde planet to complete the entire cycle of movement from the furthest point of its retrograde back to the point it left before it went retrograde.
Those who do embrace this idea believe that retrograde effects begin as soon as the planet passes the pre-retrograde shadow point, that is, the point that the planet will retrograde back to, and do not completely fade away until the planet has reached the post-retrograde shadow point, that is, the point the planet left as it began retrograde.
For example: The planet in question is moving along direct through Point A to Point B. It then goes retrograde back to Point A, then it goes direct and moves back to Point B. Point A would be the pre-retrograde shadow point and Point B would be the post-retrograde shadow point.
According to this theory, retrograde influences begin to take affect as soon as the planet passes through Point A and do not completely fade away until the planet passes through Point B.
This idea was only developed in the 1980s and has not been embraced by all astrologers.