What is the standard for determining indefiniteness in patent claims?

The standard for determining indefiniteness in patent claims is whether those skilled in the art would understand what is claimed when the claim is read in light of the specification. As stated in the MPEP:

“A decision on whether a claim is indefinite under 35 U.S.C. 112(b) or pre-AIA 35 U.S.C. 112, second paragraph requires a determination of whether those skilled in the art would understand what is claimed when the claim is read in light of the specification.”

This standard is based on the perspective of a person of ordinary skill in the relevant art, considering the claim language, specification, and prosecution history.

Claim Terms Must Be Understandable to Skilled Artisans MPEP 2173.02 Informative
Claim Language Must Be Clear and Definite MPEP 2173.02 Informative
Claim Must Inform Artisans with Certainty MPEP 2173.02 Informative
Claims Must Be Clear and Definite During Prosecution MPEP 2173.02 Informative
Claim Must Clearly Define Invention MPEP 2173.02 Recommended