either a pair of images to be viewed with glasses, or a single tiled image viewed by squinting. In both cases, the different views of each eye are "sub-mentally" compared to give the impression of a three-dimensional object emerging out of the picture.
Details and history about 3D glasses, Random Dot Stereograms and magic images
Examples (both the tiles and the "hidden" images are my own creations):
homage to Vaserely