Indefiniteness

Patent indefiniteness is an invalidity defense under 35 U.S.C. § 112(b) (formerly § 112 ¶ 2). A claim is invalid for indefiniteness if it fails to particularly point out and distinctly claim the subject matter of the invention. The practical effect: a claim is invalid if it does not tell a person of ordinary skill in the art, with reasonable certainty, the scope of the claimed invention. Courts analyze indefiniteness after claim construction — if the claim cannot be construed with reasonable certainty even using intrinsic evidence (specification, prosecution history), it is invalid.

Nautilus, Inc. v. Biosig Instruments, Inc. (S.Ct. 2014) rejected the Federal Circuit's prior 'insolubly ambiguous' standard as too permissive. The Supreme Court held that a patent claim is invalid for indefiniteness if its language, read in light of the specification and prosecution history, fails to inform, with reasonable certainty, those skilled in the art about the scope of the invention. 'Reasonable certainty' requires more than a claim that is technically not insolubly ambiguous — the claim must provide meaningful notice of the scope. After Nautilus, the Federal Circuit applies this standard but still finds claims definite when the intrinsic record (specification, figures, prosecution history) provides guidance even for otherwise vague terms.

Common indefiniteness issues: (1) TERMS OF DEGREE — 'substantially,' 'approximately,' 'about,' 'generally' — permissible if the specification provides a standard for measuring the degree (e.g., 'about 5% means within 2%'); indefinite if no guidance; (2) RELATIVE TERMS WITHOUT REFERENCE POINT — 'high speed,' 'small size,' 'improved' without a baseline; (3) FUNCTIONAL CLAIM LANGUAGE — claiming a result without structural bounds (but means-plus-function claims under § 112(f) are construed to cover only structures in the specification); (4) COINED OR UNDEFINED TERMS — inventor-defined terms without adequate specification support; (5) INCONSISTENT CLAIM SCOPE — where the claims can only be satisfied by contradictory interpretations; (6) UNCLEAR ANTECEDENT BASIS — using 'the widget' without prior introduction as 'a widget'.

All three are § 112 requirements but they target different failures: (1) INDEFINITENESS (§ 112(b)): the CLAIMS themselves fail to define the scope with reasonable certainty. The question is whether the claim boundary is knowable. (2) ENABLEMENT (§ 112(a)): the SPECIFICATION fails to teach how to make and use the full scope of the claim without undue experimentation. The question is whether the inventor taught the how. (3) WRITTEN DESCRIPTION (§ 112(a)): the SPECIFICATION fails to show the inventor actually possessed the claimed invention at the time of filing. The question is whether the inventor had it when filing. A claim can be definite but still fail enablement or written description — these are independent requirements.

Indefiniteness is most commonly raised as a defense in patent infringement litigation. It is typically briefed as part of claim construction (Markman) proceedings, because indefiniteness is analyzed as a legal question by the court (not the jury). After Nautilus, defendants frequently challenge claims containing terms of degree, functional language, or internally inconsistent scope. In IPR and PGR proceedings before the PTAB, indefiniteness cannot be raised as a ground for IPR (which is limited to § 102 and § 103 art-based invalidity); but it can be raised in PGR (post-grant review) under § 112.