http://en.wikipedia.org/wiki/Prior_art
Prior art (also known as state of the art, which also has other meanings, or background art[1]), in most systems of patent law,[2] constitutes all information that has been made available to the public in any form before a given date that might be relevant to a patent's claims of originality. If an invention has been described in prior art, a patent on that invention is not valid.
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