The Kanizsa Triangle illusion was first described in 1955 by an Italian psychologist named Gaetano Kanizsa. In the illusion, a white equilateral triangle can be seen in the image even though there is not actually a triangle there. The effect is caused by illusory or subject contours.
Gestalt psychologists use this illusion to describe the law of closure, one of the gestalt laws of perceptual organization. According to this principle, objects that are grouped together tend to be seen as being part of a whole. We tend to ignore gaps and perceive the contour lines in order to make the image appear as a cohesive whole.
Kanizsa, G. (1955). Margini quasi-percettivi in campi con stimolazione omogenea. Rivista di Psicologia 49(1): 7–30