Stereogram, or people often tell you 3D- image, is a kind of image at which you look, with a technique, you can see a sunken region inside the image.
A stereogram is an optical illusion of depth created from flat, two-dimensional image or images. Originally, stereogram referred to a pair of stereo images which could be viewed using stereoscope. Other types of stereograms include anaglyphs and autostereograms.
Stereogram was discovered by Charles Wheatstone in 1838. He found an explanation of binocular vision which led him to construct a stereoscope based on a combination of prisms and mirrors to allow a person to see 3D images from two 2D pictures.
Oliver Wendell Holmes, Sr. invented an improved form of stereoscope in 1861, which had no mirrors and was inexpensive to produce. These stereoscopes were immensely popular for decades.
Stereograms were re-popularized by the creation of autostereogram on computers, where a 3D image is hidden in a single 2D image, until the viewer focuses the eyes correctly. The Magic Eye series is a popular example of this. Magic Eye books refer to autostereograms as stereograms, leading most people to believe that the word stereogram is synonymous to autostereogram.
Salvador Dalí created some impressive stereograms in his exploration in a variety of optical illusions. One of the most common ways to create a stereogram is to hold both of your index fingers in front of you with the tips touching and cross your eyes to create the illusion that you have an extra finger.