1. The impression of a vivid sensation retained by the retina of the eye after the cause has been removed; also extended to impressions left of tones, smells, etc.
2. A visual image that remains briefly after light stimulation has ended.
3. An optical phenomenon in which the eye's nerves continue to convey an image after an initial image is no longer being viewed.

Typically, the afterimage appears as a likeness of the initial image, except that each of its colors is the complement to those in the initial image. Sometimes called a complementary afterimage or a photogene.

You can experience an after image with the following presentation:

Look at the + in the middle of the blue figure above for 15-30 seconds. Then look at the tiny + in the center of the white square on the right.

The reversal of the color you see is an example of an afterimage. Notice how long the afterimage continues before it disappears.