What is the meaning of artists proof?

Artists proof is a term used by all kind of printed artworks, usually consisting in the first resulted impressions that came out of the stone, lithographic, silkscreen, Giclee, etc. And the artist keeps, not affecting the numbered editions, usually are less than five copies and serious artists let this information to be known by the art dealers and art collectors. Example: this edition consist of 25 pieces with 3 artists proof.