Since at least 1998, the Federal Circuit has held that when a patent uses the word "means" in a claim element, that creates a "rebuttable presumption" that the limitation is a means-plus-function limitation to which Paragraph 6 of 35 U.S.C. § 112 applies. The converse also creates a presumption—if the claim fails to use the word "means," it is presumed not to be a means-plus-function element.

Starting with its 2004 decision in Lighting World, Inc. v. Birchwood Lighting, Inc., 382 F.3d 1354 (Fed. Cir. 2004), the Federal Circuit's opinions began characterizing the presumption that flowed from the absence of the word "means" as "a strong one that is not readily overcome," thereby making the presumption virtually conclusive in many, if not most, cases.

However, on June 16, 2015, the Federal Circuit issued a revised opinion inWilliamson v. Citrix Online, LLC, No. 13-1130, overruling prior decisions characterizing this presumption as "strong" and holding that the court "will apply the presumption as we have done prior to" its decision in Lighting World, "without requiring any heightened evidentiary showing." The court also "overrule[d] the strict requirement of 'a showing that the limitation essentially is devoid of anything that can be construed as structure.'"

Instead, the standard is now to be "whether the words of a claim are understood by persons of ordinary skill in the art to have a sufficiently definite meaning as the name for structure." The presumption that a claim term lacking the word "means" does not invoke paragraph 6 of section 112, therefore, can be overcome "if the challenger demonstrates that the claim term fails to 'recite sufficiently definite structure' or else recites 'function without reciting sufficient structure for performing that function.'" The converse rebuttable presumption, that a claim limitation containing the word "means" is to be construed as a means-plus-function limitation, remains unaffected.

Because it involved overruling prior precedent, the Federal Circuit reached this decision by resolving this limited legal issue with the full en banc court. Judge Newman, alone, dissented.

This decision will have important consequences not only in litigation but in claim drafting as well, particularly in areas like computer software and the electronic arts, where means-plus-function claiming is prominently used.