Base58

Base58 is a family of binary-to-text encodings which use an alphabet of 58 symbols. The only reason for the number 58 in particular is that it's the number of symbols available if you start with the 62 ASCII alphanumeric characters, and disallow the visually-ambiguous I, l, O, and 0.

A version of it called Base58Check is used to encode Bitcoin addresses.

Links

 * Wikipedia: Base58
 * Bitcoin wiki: Base58Check encoding