An inter partes review is a trial proceeding first instituted in 2012 that is conducted by the patent review board to ascertain the validity of one or more claims of a patent based on prior art consisting of patents or printed publications. This is a modification of the inter partes reexamination first instituted in 1999, but fixes a number of limitations and problems with that procedure.
The review is called by a third party (a person who is not the patent owner, has not previously filed a civil action challenging the validity of a claim of the patent, and has not been served with a complaint alleging infringement of the patent more than 1 year prior) and must provide evidence that there is a reasonable likelihood that the petitioner would prevail with respect to at least one claim challenged. The patent holder has the ability to make supporting declarations.
As a result of the changes, this has become a quicker, and thus more popular method, to remove a patent from preventing product introduction. It only requires a preponderance standard rather than clear and convincing evidence.