Prior art

Loosely, all information that is publicly available before someone claims to invent something.  Patents are only allowed for things that are novel and nonobvious.  If the prior art contained a description of the supposed invention, it usually cannot be novel.  If the prior art contained enough information that the invention would have been obvious to someone of ordinary skill in the relevant field, a patent also cannot issue.  See 35 U.S.C. §§ 102(a), 103(a).