An ambigram is a typographical design or artform that may be read as one or more words not only in its form as presented, but also from another viewpoint, direction, or orientation. The words readable in the other viewpoint, direction or orientation may be the same or different from the original words. 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.

Ambigrams are exercises in graphic design that play with optical illusions, symmetry and visual perception. Some ambigrams feature a relationship between their form and their content. Ambigrams usually fall into one of several categories:

Rotational
A design that presents several instances of words when rotated through a fixed angle. This is usually 180 degrees, but rotational ambigrams of other angles exist, for example 90 or 45 degrees. The word spelled out from the alternative direction(s) is often the same, but may be a different word to the initially presented form. A simple example is the lower-case abbreviation for “Down”, dn, which looks like the lower-case word up when rotated 180 degrees.
Mirror-image
A design that can be read when reflected in a mirror, usually as the same word or phrase both ways. Ambigrams that form different words when viewed in the mirror are also known as glass door ambigrams, because they can be printed on a glass door to be read differently when entering or exiting.
Figure-ground
A design in which the spaces between the letters of one word form another word.
Chain
A design where a word (or sometimes words) are interlinked, forming a repeating chain. Letters are usually overlapped meaning that a word will start partway through another word. Sometimes chain ambigrams are presented in the form of a circle.
Space-filling
Similar to chain ambigrams, but tile to fill the 2-dimensional plane.
Spinonym
An ambigram in which all the letters are made of the same glyph, possibly rotated and/or inverted. WEB is an example of a word that can easily be made into a spinonym. Previously called rotoglyphs or rotaglyphs.[14]
Fractal
A version of space-filling ambigrams where the tiled word branches from itself and then shrinks in a self-similar manner, forming a fractal. See Scott Kim’s fractal of the word TREE for an animated example.
3-dimensional
A design where an object is presented that will appear to read several letters or words when viewed from different angles. Such designs can be generated using constructive solid geometry.
Perceptual shift (also called an oscillation)
A design with no symmetry but can be read as two different words depending on how the curves of the letters are interpreted.
Natural
A natural ambigram is a word that possesses one or more of the above symmetries when written in its natural state, requiring no typographic styling. For example, the words “dollop”, “suns” and “pod” form natural rotational ambigrams. In some fonts, the word “swims” forms a natural rotational ambigram. The word “bud” forms a natural mirror ambigram when reflected over a vertical axis. The words “CHOICE” and “OXIDE”, in all capitals, form natural mirror ambigrams when reflected over a horizontal axis. The word “TOOTH”, in all capitals, forms a natural mirror ambigram when its letters are stacked vertically and reflected over a vertical axis.
Symbiotogram[citation needed]
An ambigram that, when rotated 180 degrees, can be read as a different word to the original.
Multi-lingual
An ambigram that can be read one way in one language and another way in a different language. Multi-lingual ambigrams can exist in all of the various styles of ambigrams, with multi-lingual perceptual shift ambigrams being particularly striking.
(wikipedia.com)

So if you have a tattoo that can be described as an Ambigram, you are looking at getting an Ambigram design, or found an Ambigram design/tattoo that you like, post it here.