Illusory contour is stranger than it sounds.

An illusory contour or subjective contour is a visual illusion that evokes the perception of an edge without a luminance or color change across that edge.

An illusory contour or subjective contour is a visual illusion that evokes the perception of an edge without a luminance or color change across that edge. Illusory brightness and depth ordering often accompany illusory contours. Friedrich Schumann is often credited with the discovery of illusory contours around the beginning of the 20th century, but they are present in art dating to the Middle Ages. Gaetano Kanizsa’s 1976 Scientific American paper marked the resurgence of interest in illusory contours for vision scientists.