The Artists Proof (AP) is the first print or a subset of prints made, based on the original painted or printed work. In many cases, when a work of art has not had an Limited Edition release - this is the only official signed & numbered version available other than the Original itself.
In the event of a Limited Edition release of the print, the AP is the standard that all subsequent prints must meet. AP's are therefore very rare.
There are usually no less than 3 and no more than 20 of an Artist Proof produced normally accounting for 5-10% of the edition count.