The Kanizsa triangle is an optical illusion first discovred by the Italian psychologist Gaetano Kanizsa in 1955. In the accompanying figure a white equilateral triangle is perceived, but in fact none is drawn.
This effect is known as a subjective or "illusory contour." Also, the nonexistent white triangle appears to be brighter than the surrounding area, but in fact it has the same brightness as the background.
Another contour illusion is the Ehrenstein illusion.