An ambigram as defined by Wikipedia "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."
To simply put, an ambigram is a typographical creation where the word reads the same when upside down, or flips to create a whole new word. Ambigrams are often very sophisticated and very imaginative typographical style of visual design.
For another ambigram examples, visit the ambigram gallery at ambigram.com - http://www.ambigram.com/category/ambigram-gallery-designs/default.htm .