On June 2, 2014, in Nautilus, Inc. v. Biosig Instruments, Inc., the US Supreme Court vacated the US Court of Appeals for the Federal Circuit's decision that a patent was not indefinite under 35 U.S.C. § 112, ¶2 (now 35 U.S.C. § 112(2)) (No. 13-369, (S. Ct. June 2, 2014)). The Supreme Court held that a claim is invalid for indefiniteness if it fails to inform a skilled artisan with reasonable certainty of the invention's scope when read in light of the specification and prosecution history. The Supreme Court rejected the Federal Circuit's narrower test that a claim is only indefinite when it is "insolubly ambiguous."

Biosig owns US Patent No. 5,337,753 ('753 patent) covering a heart-rate monitor for use during exercise. The patent claims a cylindrical bar containing a live and common electrode "mounted ... in spaced relationship with each other" to minimize interference and improve the heart-rate measurement. Biosig sued Nautilus, Inc. for patent infringement in the US District Court for the Southern District of New York. The parties dismissed the case without prejudice after Nautilus sought reexamination of the patent at the USPTO. In 2010, the USPTO confirmed the invention's patentability.

Biosig then reinstituted its infringement suit against Nautilus. After the district court issued its claim construction order, Nautilus moved for summary judgment that the claimed electrodes "mounted ... in spaced relationship with each other" were indefinite under 35 U.S.C. § 112, ¶ 2. The district court granted Nautilus' motion for summary judgment, finding that the patent did not disclose how to determine the appropriate spacing between the electrodes.

On appeal, the Federal Circuit reversed and found that the term was definite, explaining that a claim is only indefinite when it is either:

In a unanimous decision, the Supreme Court rejected the Federal Circuit's indefiniteness test and explained that:

To balance these concerns, the Supreme Court held that a claim is invalid for indefiniteness if it fails to inform a person skilled in the art of the invention's scope with reasonable certainty when read in light of the specification and prosecution history. The Supreme Court remanded the case so the Federal Circuit can decide whether the '753 patent is indefinite under the appropriate standard.

The Supreme Court's decision should make it easier for accused infringers to prevail on indefiniteness defenses and highlights the importance of fact and expert discovery concerning this issue.

Update: On April 27, 2015, the Federal Circuit in Biosig Instruments, Inc. v. Nautilus, Inc., maintained its reversal of the district court's determination that Biosig's patent claims were indefinite (No. 2012–1289, (Fed. Cir. Apr. 27, 2015)). The Federal Circuit held on remand that the "spaced relationship" phrase complied with 35 U.S.C. § 112 because it informed those skilled in the art about the scope of the invention with reasonable certainty.