Indefiniteness

In Nautilus v. Biosig (2014) the Supreme Court rejected the Federal Circuit's formulation that a claim passes if it is "amenable to construction" and not "insolubly ambiguous", holding that such formulations lack the precision the statute demands¹. The Court also said the definiteness requirement must take into account the inherent limitations of language, and that, at the same time, a patent must be precise enough to afford clear notice of what is claimed¹. Nautilus construed the earlier wording of the statute, which referred to what "the applicant regards" as the invention¹. The wording quoted above, "the inventor or a joint inventor", applies to applications filed on or after 16 September 2012, when the 2011 amendment took effect².

Claim limitations written as a means or step for performing a specified function are read under 35 U.S.C. 112(f) to cover the corresponding structure, material or acts described in the specification and equivalents thereof². How that interacts with definiteness is covered on Means-plus-function claim.

Where indefiniteness can be raised in the US depends on the proceeding. In a civil action, invalidity for failure to comply with any requirement of section 112 is a defense that must be pleaded, subject to a carve-out for best mode (35 U.S.C. 282(b)(3)(A))⁵. A patent is presumed valid, and the burden of establishing invalidity rests on the party asserting it⁵. The Supreme Court has held that section 282 requires an invalidity defense to be proved by clear and convincing evidence⁶. In a post-grant review, a petitioner may raise any ground that could be raised under paragraph (2) or (3) of section 282(b), which includes section 112, but post-grant review is available only for patents subject to the first-inventor-to-file provisions of the AIA, and the petition must be filed within 9 months after grant⁷. An inter partes review is different: it is limited to grounds under sections 102 or 103 based on patents or printed publications, so indefiniteness is not a ground there⁸. This list of proceedings is not exhaustive. See Post-grant review and Inter partes review.

The MPEP says the primary purpose of the definiteness requirement is to ensure that the scope of the claims is clear, so that the public is informed of the boundaries of what constitutes infringement⁴. It also records the Nautilus standard for courts⁴.

The Office does not interpret claims during examination in the same way as courts. It gives claims their broadest reasonable interpretation in an effort to establish a clear record of what the applicant intends to claim, and the MPEP says this may effectively result in a lower threshold for ambiguity than a court's determination⁴. If, under that interpretation, a person of ordinary skill would read the claim with more than one reasonable interpretation, a rejection under section 112(b) is appropriate⁴. The applicant can respond by amending the claims to make the meaning clear⁴.

The MPEP also cautions examiners against confusing breadth with indefiniteness, and says relative terminology, including terms of degree, does not automatically render a claim indefinite⁴. The MPEP page cited here is marked R-01.2024. The USPTO states that the current MPEP is the 9th edition, Revision 01.2024, and that changes made after 31 January 2024 are not incorporated in it⁹.

Article 84 EPC requires the claims to define the matter for which protection is sought, to be clear and concise and to be supported by the description³. Support by the description is a separate limb from clarity. The EPO Guidelines (2026 edition) say clarity is of the utmost importance for reasons of legal certainty, that there should be no doubt as to which subject-matter is covered by the claims, and that the skilled person should be able to establish the scope of the claim without undue burden¹⁰. The requirement applies to independent and dependent claims alike, and to the claims as a whole, and the correct response to a lack of clarity is amendment¹⁰.

On relative terms, the Guidelines say terms such as "thin", "wide" or "strong" constitute a potentially unclear element because their meaning may change depending on the context¹¹. The MPEP takes a different starting point: relative terminology does not automatically render a claim indefinite⁴.

After grant, the position differs from the US. Article 100 EPC lists the grounds on which an opposition may be filed, and lack of clarity is not among them¹². The Guidelines state that clarity is not a ground for opposition, and that claims of a patent as amended in opposition may be examined for compliance with Article 84 only when, and only to the extent that, an amendment introduces non-compliance (G 3/14)¹³. Article 138(1) EPC, which lists the only grounds on which a European patent may be revoked with effect for a Contracting State, likewise does not include lack of clarity¹⁴.

The Supreme Court decision below is the source of the US standard. It is an example of how the term is applied, and says nothing about either party beyond the opinion itself.

The Court replaced the Federal Circuit's "insolubly ambiguous" formulation with the rule that a patent is invalid for indefiniteness if its claims fail to inform, with reasonable certainty, those skilled in the art about the scope of the invention. It expressed no opinion on the validity of the patent in suit and remanded for the lower court to apply the standard1.