According to common definition, an optical illusion is:

• A visually perceived image that is deceptive or misleading.
• Visual perception of a real object in such a way as to misinterpret its actual nature.
• An optical phenomenon that results in a false or deceptive visual impression (an eye fooler).

Yet, despite which definition is preferred, the phenomenon of an optical illusion remains the same.
 

Rapid Colored Afterimage
 

A perfect example of how your eyes can be deceived through images is through rapid colored afterimages. (This visual illusion is just one of 55 "Optical Illusions and Visual Phenomena.")

Let your gaze rest on the central fixation cross, but observe with your “inner eye” the patches just when they disappear. With the rapid colored afterimage illusion in particular, the viewer will notice a circle of blue-violettish/ magenta dots--one of which disappears briefly--rotating around a circle. While there is absolutely no green in this circle of dots, with good fixation, the viewer will begin to see a strong greenish color whenever the magenta dot disappears. With steady focus, eventually, it will appear as though there is a rotating green spot.

But that's not all. If the viewer's gaze is really steady, all of the magenta dots will disappear, leaving nothing but the rotating green spot!