Wikipedia
The term artistamp (a portmanteau of the words "artist" and "stamp") or artist's stamp refers to a postage stamp-like art form used to depict or commemorate any subject its creator chooses. Artistamps are a form of Cinderella stamps in that they are not valid for postage, but they differ from forgeries or bogus Illegal stamps in that typically the creator has no intent to defraud postal authorities or stamp collectors.
Artistamp creators often include their work on legitimate mail, alongside valid postage stamps, in order to decorate the envelope with their art. In many countries this practice is legal, provided the artistamp is not passed off as or likely to be mistaken for a genuine postage stamp. When so combined (and sometimes, less strictly speaking, even when not so) the artistamp may be considered part of the mail art genre.
Irony, satire, humor, eroticism and subversion of governmental authority are frequent characteristics of artistamps. Artists may leverage the expectation of official endorsement that necessarily inheres in governmentally-issued postage for the purpose of shocking or subverting viewers' expectations, with such actions typically representing a specific political and artistic motive. Other practitioners are content to depict more homey subjects like kittens and family members. Some artists use the form to create fantasy stamps for their own postal administrations or countries – in many cases thereby developing or complementing an imaginary governmental system.