Definition: An afterimage is a type of visual illusion in which an image continues to appear for a brief period of time even though it is no longer present in the visual field. You can see an example of this yourself by trying the following demonstration. Stare at the image to the right for about 30 seconds and then shift your vision quickly to a plain white background. If you have performed the illusion successfully, you will continue to see the blue and red shamrock image as a yellow and green afterimage.
Click the following link to learn more about how afterimages work.