Definiteness under § 112(b)
A patent claim must inform a person of ordinary skill in the art, with reasonable certainty, of the scope of the invention. The Supreme Court replaced the Federal Circuit's earlier "insolubly ambiguous" formulation with this reasonable-certainty standard in Nautilus v. Biosig, raising the bar for clarity in claim drafting and giving accused infringers a sharper tool against vague terms.
The rule
Under 35 U.S.C. § 112(b), the specification "shall conclude with one or more claims particularly pointing out and distinctly claiming the subject matter which the inventor or a joint inventor regards as the invention." A claim fails this requirement, and is therefore invalid for indefiniteness, if "its claims, read in light of the specification delineating the patent, and the prosecution history, fail to inform, with reasonable certainty, those skilled in the art about the scope of the invention." Nautilus, Inc. v. Biosig Instruments, Inc., 572 U.S. 898, 901 (2014).
Definiteness is a doctrinal balance. Some imprecision is unavoidable; perfect clarity is not required. The Court in Nautilus recognized that absolute precision is unattainable, while insisting that the public must be able to discern, with reasonable certainty, what the patent forbids and what it permits.
Statutory and constitutional source
Section 112(b) of the Patent Act, before the AIA renumbering known as § 112, ¶ 2, requires that claims particularly point out and distinctly claim the invention. The provision serves the public-notice function central to the patent system: a patent's claims set the boundaries of the right to exclude, and competitors must be able to read those boundaries to plan their own conduct.
The Supreme Court has repeatedly emphasized this notice function. In General Electric Co. v. Wabash Appliance Corp., 304 U.S. 364 (1938), and again in Nautilus, the Court connected definiteness to the constitutional and statutory premise that exclusive rights are granted in exchange for clear public disclosure.
The framework the courts apply
The reasonable-certainty standard
Before Nautilus, the Federal Circuit applied an "insolubly ambiguous" test, under which a claim survived definiteness review if it was amenable to construction, however difficult, and was not insolubly ambiguous. The Supreme Court rejected that formulation as inconsistent with § 112(b)'s public-notice function, holding that a claim is indefinite unless it "inform[s] those skilled in the art about the scope of the invention with reasonable certainty." Nautilus, 572 U.S. at 910.
The standard is objective and is applied in light of the specification and the prosecution history, from the perspective of a person of ordinary skill in the art at the time the patent was filed. The skilled artisan's ability to give meaning to a term is informed by extrinsic evidence such as expert testimony and dictionaries, but the principal sources are the intrinsic record. The Federal Circuit has applied Nautilus in cases involving terms of degree, functional language, and ranges with unstated reference points. See, e.g., Interval Licensing LLC v. AOL, Inc., 766 F.3d 1364 (Fed. Cir. 2014) ("unobtrusive manner that does not distract a user" indefinite); Dow Chemical Co. v. Nova Chemicals Corp., 803 F.3d 620 (Fed. Cir. 2015) (claim term referencing slope of strain hardening coefficient indefinite where multiple methods of measurement gave different results).
Terms of degree
Terms of degree — "substantial," "about," "near" — are not categorically indefinite. They are permissible when the specification provides some standard for measuring the degree. Where the specification supplies neither examples nor objective benchmarks, terms of degree may fail the reasonable-certainty test. The analysis is fact-intensive and turns on the level of skill in the art and the nature of the technology.
Means-plus-function claims under § 112(f)
Section 112(f) (formerly § 112, ¶ 6) permits a patentee to express a claim element as a "means" or "step" for performing a specified function without reciting the structure that performs that function. When § 112(f) applies, the claim element is construed to cover the corresponding structure, material, or acts described in the specification and equivalents thereof. The trade-off is significant: § 112(f) lets the drafter use shorthand functional language but limits the claim to the disclosed embodiments and their equivalents.
If a means-plus-function claim recites a function but the specification fails to disclose corresponding structure, the claim is indefinite. Aristocrat Technologies Australia Pty Ltd. v. International Game Technology, 521 F.3d 1328 (Fed. Cir. 2008). For computer-implemented means-plus-function claims, the corresponding structure is the algorithm disclosed in the specification; absence of an algorithm renders the claim indefinite. WMS Gaming Inc. v. International Game Technology, 184 F.3d 1339 (Fed. Cir. 1999).
Williamson v. Citrix and the means-plus-function presumption
For decades, the Federal Circuit applied a "strong" presumption that absence of the word "means" took a claim out of § 112(f). The court overruled that doctrine en banc in Williamson v. Citrix Online, LLC, 792 F.3d 1339 (Fed. Cir. 2015), holding that the presumption "is not strong" and that the proper inquiry is whether the claim term, viewed in light of the specification, would have been understood by a person of ordinary skill to recite sufficiently definite structure. Generic terms — "module," "device," "mechanism," "element," "unit" — when coupled with functional language and lacking structural connotation, may invoke § 112(f) even without the word "means."
After Williamson, district courts and the PTAB more readily find that nominally non-means terms are subject to § 112(f), with the result that the term is construed to cover only disclosed corresponding structure or, if no structure is disclosed, is indefinite. The decision shifted significant scope away from claims that had relied on functional black-box terms.
Burden and proof
Issued patents are presumed valid under 35 U.S.C. § 282(a), and the accused infringer must prove indefiniteness in district court by clear and convincing evidence. Microsoft Corp. v. i4i Limited Partnership, 564 U.S. 91 (2011). Indefiniteness is a question of law, which the Federal Circuit reviews de novo, with underlying factual findings reviewed for clear error after a bench trial. Teva Pharmaceuticals USA, Inc. v. Sandoz, Inc., 574 U.S. 318 (2015).
In post-grant review, the burden is preponderance of the evidence under 35 U.S.C. § 326(e). Indefiniteness may be raised in PGR but not in inter partes review, which is restricted to § 102 and § 103 grounds based on patents and printed publications. Definiteness is examined during prosecution, and the Federal Circuit has at times suggested that the standard during examination, where claims may still be amended, is more stringent than the post-issuance standard. The PTAB applies Nautilus in PGR proceedings.
Interaction with related doctrines
Definiteness is intimately tied to claim construction. The Federal Circuit's seminal Phillips v. AWH Corp., 415 F.3d 1303 (Fed. Cir. 2005) (en banc), framework supplies the interpretive tools that often determine whether a term meets reasonable certainty. Construction and definiteness are sometimes resolved together at the Markman stage; in other cases, the court constructs the term and saves the indefiniteness question for trial.
Definiteness intersects with written description and enablement. Functional claim terms that fail § 112(b) often also fail § 112(a) for lack of supporting disclosure, particularly where the claim covers a desired result without specifying how to achieve it. Definiteness can also bear on literal infringement: a vague term may be construed narrowly to preserve validity, and that narrow construction can leave the accused product outside the claim.
Definiteness has growing relevance to § 101 eligibility as well. The same functional black-box drafting that triggers definiteness scrutiny under Williamson often also raises eligibility concerns under Alice.
Practical notes
Indefiniteness is most often raised at the Markman stage as part of claim construction briefing, or at summary judgment after construction. Defendants frequently challenge terms of degree without specification anchors, functional terms whose corresponding algorithms are missing or ambiguous, and ranges with unstated reference points. Patentees defend by directing the court to specification examples, prosecution history, and expert testimony establishing how a person of ordinary skill would have understood the term.
Drafting practice has responded to Nautilus and Williamson. Modern claims more often define terms of degree by reference to disclosed test methods or numerical ranges. Software claims more often recite specific operations rather than generic "modules" or "engines," in order to avoid being recharacterized as means-plus-function under Williamson.
The Federal Circuit reviews legal definiteness rulings de novo. Practitioners should expect contested expert testimony at the district-court level on the level of ordinary skill and on whether the intrinsic record supplies reasonable certainty, and should preserve clear-error arguments on any underlying factual findings under Teva v. Sandoz.
Open questions
The application of Nautilus to terms of degree, the threshold for invoking § 112(f) under Williamson, and the necessary specificity of disclosed algorithms for computer-implemented means-plus-function claims all remain active areas of doctrinal development. The Federal Circuit's panel decisions in these areas continue to refine the boundaries, and the interaction between definiteness and eligibility under § 101 is still evolving as software claim drafting adjusts to multiple overlapping doctrines.
See also
- Written description and enablementThe companion § 112(a) disclosure requirements.
- Claim constructionDefines the scope a definiteness challenge tests.
- Subject matter eligibility under § 101Functional claims raise both eligibility and definiteness concerns.
- Literal infringementA construction adopted to preserve validity may narrow infringement.
- Phillips v. AWHThe framework for interpreting claim terms.
- Markman hearingsThe procedural setting for most definiteness disputes.
- Post-grant reviewThe PTAB forum where § 112(b) grounds may be raised.
Last reviewed: 2026