The defintion of an optical illusion is a perception of a visual stimuli that represents what isperceived in a way different from the way it is in reality. It is your mind tricking you into believing you are seeing one thing, when in fact it is something else. There are three types of Optical Illusions.
The first type is Literal Illusions. Literal Illusions create images that are completely different from the objects that create them. An example would be the three birds flying in the sky, but the way they are positioned creates the illusion of a smiley face.
The second type is Physiological Illusions. Physiological Illusions occur after excessive visual stimulation. An example would be the grid illusion, where white lines crossing on a black grid create perceptions of black dots at the intersections.
The third type is Cognitive Illusions. Cognitive Illusions are based on the natural assumption of what the viewer sees. There are four subtypes. Ambiguous Illusions are illusions that can switch between alternate perceptions. An example would be the duck- rabbit illusion. Paradox Illusions create objects that are possible in reality. Fictional Illusions are illusions only the observer can see, such as hallucinations. Distorting Illusions are misrepresentations of size or shape. An example of Distorting Illusions would be sidewalk art. An artist draws a picture on the sidewalk, but the picture becomes 3D when you observe it from a certain angle.