An anamorphosis is a deformed image that appears in its true shape when viewed in some “unconventional” way. According to Webster’s 1913 Dictionary:
A distorted or monstrous projection or representation of an image on a plane or curved surface, which, when viewed from a certain point, or as reflected from a curved mirror or through a polyhedron, appears regular and in proportion; a deformation of an image.
In one common form of anamorphosis—usually termed “oblique”— the unconventionality arises from the fact that the image must be viewed from a position that is very far from the usual in-front and straight-ahead position from which we normally expect images to be looked at.
In another common form—sometimes termed “catoptric”—the image must be seen reflected in a distorting mirror (typical shapes being cylindrical, conical and pyramidal). The Hungarian artist István Orosz has produced some beautiful examples of these.