In most patent laws, prior art or state of the art is all information that has been disclosed to the public in any form before a given date. However, under United States Patent Law, non-public prior art, such as a sale carried out in secret (e.g., under a non-disclosure agreement), qualifies as prior art in certain circumstances. In Europe, prior art does not include information kept secret, whether from trade secrecy or just a simple lack of interest in publication. (cited from en.wikipedia.org/wiki/Prior_Art)