Stereogram

A stereogram is an optical illusion of depth brought into appearance by focusing one's eyes (usually) behind or sometimes in front of (crossing one's eyes) the image one is looking at. The slight differences in vertical repetitions of figures or random dots create the illusion of depth in the 2D image, just as the slight difference in perspective between one's eyes creates the perception of depth on 3D objects and scenes.
Stereograms had been created for years before being re-popularized by the creation of random-dot stereograms on computers. Instead of the traditional two photographs of an object or scene taken from a slightly different angle (see stereoscopy), in random-dot stereograms the image is hidden until the viewer focuses the eyes correctly. The Magic Eye series is a popular example of this.
Salvador Dalí created some impressive stereograms in his exploration in a variety of optical illusions.