visual illusion

n. Any of dozens of sorts of incorrect visual impressions which can be produced through many physiological and psychological mechanisms. Some visual illusions are normal and caused by the working processes of the visual system, such as in the figure ⊥ where the vertical line appears to be longer than the horizontal line when both are the same length. Other visual illusions are caused by damage to the visual system, as when a person sees two or more images of the same object or continues to see an object after it has gone. Also called optical illusion.