BSL Physics Glossary - image retention - definition
Your brain has a memory of an image. For example, when you look at a bright light for a while, really looking straight at it, then the light gets turned off, your brain will still see an image there. As you move your head, this retained image will move too. Another example is if you watch a film where about a spaceship moving through space with thousands of stars whizzing past. If that image then stops, you may then experience a sort of ghost image of stars moving away from you. It's not a real image. Your brain has a memory of the original image which it has held on to, or retained.