The word ambigram was coined by Douglas Hofstadter, an American scholar of cognitive science, best known as the Pulitzer Prize-winning author of the book Gödel, Escher, Bach.
“An ambigram is a visual pun of a special kind: a calligraphic design having two or more (clear) interpretations as written words. One can voluntarily jump back and forth between the rival readings usually by shifting one’s physical point of view (moving the design in some way) but sometimes by simply altering one’s perceptual bias towards a design (clicking an internal mental switch, so to speak). Sometimes the readings will say identical things, sometimes they will say different things.”
Hofstadter describes an ambigram as a “calligraphic design that manages to squeeze two different readings into the selfsame set of curves.”
Different ambigram artists (sometimes called ambigramists) may create distinctive ambigrams from the same words, differing in both style and form.
Although the term is recent, the existence of mirror ambigrams has been attested since at least the first millennium. They are generally palindromes stylized to be visually symmetrical.
The earliest known non-natural rotational ambigram dates to 1893 by artist Peter Newell. Although better known for his children’s books and illustrations for Mark Twain and Lewis Carroll, he published two books of reversible illustrations, in which the picture turns into a different image entirely when flipped upside down.
Rotational faces as optical illusion are very old, since metal coins with reversible figures were produced in 1550, and maybe earlier. In this perspective, a 180° rotational ambigram “¡OHO!” was published in 1946 for the cover of a book gathering reversible drawings by Rex Whistler.