Under U.S. patent law, an invention lacks novelty if it was publicly known in the U.S. prior to the date it was (possibly independently) invented by the applicant for a patent. [35 U.S.C. 102(a)]. This rule and certain others in 35 U.S.C. 102 address the novelty of the invention, i.e., whether the invention was in the public domain in the U.S. before the inventor allegedly invented it. These novelty rules reflect the unusual nature of the U.S. patent system which awards patents to the "first-to-invent" rather than the "first to file" a patent application.
The knowledge to which this rule refers must be a complete and enabling description of the invention, and it must exist in the U.S. (or its territories or possessions). Knowledge of the invention outside of the U.S., e.g., in Mexico, is of no consequence. The knowledge must be publicly available and the prior art invention must be essentially the same as ("patentably indistinct" from) the claimed invention, though the inventions need not be identical. An invention that is not novel is termed "anticipated" by the prior art.
The knowledge must have been available to the public in the U.S. before the applicant's date of invention. The Examiner will initially assume that the applicant's date of invention is the same as the effective U.S. filing date. If knowledge of the invention was introduced into the U.S. between the time of date of the invention and the filing date, it may become necessary to prove that the date of invention occurred before the introduction of knowledge occurred.
During prosecution of a U.S. patent application, the Examiner may assert that he/she possesses personal knowledge that anticipates a claimed invention. In that case, the applicant can (and should) call for the Examiner to present evidence or citations that support the assertion in the form of a formal, written document (called an affidavit or declaration). Failure to do so can render the assertion that the alleged knowledge existed in the U.S. a "fact" as far as prosecution of the application is concerned.