(n.) an artistic technique in which a hidden image is revealed when viewed in a certain way
Mirror anamorphosis is a fairly obscure art technique in which a distorted image is “hidden” inside a larger artwork, and can only be revealed by placing of a specialized mirror (called an anamorphoscope) in or alongside it:
Everything from sculptures to murals to signposts can be made anamorphically. But this being mirror anamorphosis specifically, over on Twitter we illustrated the term with a gif of an extraordinary artwork by the equally extraordinary Hungarian artist István Orosz: when a cylindrical mirror is placed inside his picture, it reveals a portrait of the author Jules Verne, whose 1874 novel The Mysterious Island was Orosz’s inspiration. (And if you think that’s impressive, seriously—take a look at some of Orosz’s other work.)
Etymologically, anamorphosis isn’t quite as enigmatic as the artworks it describes. First used in English in the early 1700s, it simply means “transformation” or “reformation”, and is built from Greek units meaning “back” or “again” (as in anabaptist), and “form” or “shape” (as in morphology, or Morpheus, the god of sleep whose name literally means “maker of shapes”).
A distortion of perspective or appearance, incidentally, can be called an anamorphism, while something that is anamorphous has been pushed or distorted out of shape.