An ambigram may be defined as an “amphibian” word (or group of words) occupying at the same time two different reading planes of a page. The distinctive feature of an ambigram is that it remains unchanged even when inverted, or reveals the coexistence of a second word when either the page is reoriented (by rotations or reflections) or when you simply change your vantage point.
Types of Ambigrams:
The most common type is the rotational ambigram. That is a word or phrase written in such a way that it appears identical or completely different when it is read upside-down.
Also known as bilateral or mirror-image symmetry, this refers to words that can be read normally before and after reflection in a mirror.
Any ambigram where both the positive (figure) and negative (ground) space read as words.
Ambigrams on my Behance gallery