In patent law, anticipation refers to the prior invention or disclosure of the claimed invention by another, or the inventor's own disclosure of the claimed invention by publication, sale, or offer to sell prior to the inventor's application for a patent. In other words, if someone else has known about or used the invention before the patent applicant applies for a patent, that patent applicant will not be entitled to a patent.
Anticipation is a grounds for invalidating or rejecting a patent because it means that the claimed invention lacks novelty. Patent invalidity based on lack of novelty, or anticipation, requires that the invention was known or used by others before it was invented by the patentee.
Definition from Nolo’s Plain-English Law Dictionary
A situation in which an invention is too similar to an earlier invention to be considered new (or novel). Because novelty is a requirement for patentability, anticipated inventions are not patentable. An invention is usually anticipated by 1) prior publications (a news article, trade journal article, academic thesis, or prior patent), 2) prior inventions (if all significant elements of the later invention are found in an earlier one prior to the date of invention or the applications filing date), 3) placing the invention on sale more than one year prior to an applications being filed, or 4) public use or display of the invention more than a year prior to filing the patent application. (See also: one-year rule, prior art)