The Kanizsa triangle is a famous optical illusion that was first described by the Italian psychologist Gaetano Kanizsa in 1955. In this figure we "see" a white equilateral triangle 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. In fact it has the same brightness as the background.
Another contour illusion is the Ehrilich illusion (2).