Which is a Ambigram?

This is called the words or phrases that can be read on more than one position due to certain symmetry presented letters on its vertical axis, others on its horizontal axis, and others on both.

For example, presents a vertical symmetry, while the D presents the horizontal and the X on both axles. This allows fingers to write on a mirror and read the same word reflected.

Another cause is the use of upper or lower case: in this case, the symmetrical horizontal h would be p, and the u n, while the b would be symmetrical vertically below the d.

An example are the words bum (lazy) and mud (mud), equal in vertical but with different meanings depending on the side of the mirror it reads.