Ambigram

From the longer Wikipedia page which has a range of examples.
An ambigram is a word, art form or other symbolic representation, whose elements retain meaning when viewed or interpreted from a different direction, perspective, or orientation. The meaning of the ambigram may either change, or remain the same.
Douglas R. 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 completely different ambigrams from the same word or words, differing in both style and form.
The earliest known non-natural ambigram dates to 1893 by artist Peter Newell.
The earliest known published reference to the term "ambigram" was by Hofstadter, who attributed the origin of the word to conversations among a small group of friends during 1983–1984.
There is various coverage on the web, including ambigram generators.
Category:Reference