What is the standard for indefiniteness in patent claims?

This page is an FAQ based on guidance from the Manual of Patent Examining Procedure. It is provided as guidance, with links to the ground truth sources. This is information only: it is not legal advice.

The standard for indefiniteness in patent claims is based on the “reasonable certainty” test established by the Supreme Court. As stated in MPEP 2173.02:

“A claim is indefinite when it contains words or phrases whose meaning is unclear. In re Packard, 751 F.3d 1307, 1314 (Fed. Cir. 2014). The test for definiteness under 35 U.S.C. 112(b) is whether the claim, read in light of the specification and the prosecution history, would inform those skilled in the art about the scope of the invention with reasonable certainty.”

This standard requires that the claims, when read in light of the specification and prosecution history, must provide clear warning to others as to what constitutes infringement of the patent. The definiteness requirement strikes a balance between the inherent limitations of language and the need for clear patent boundaries.