If a patent claim is indefinite, it is invalid. The key question, of course, is what constitutes indefiniteness. Well, patent draftspersons, rejoice. In a rare, precedential opinion, Ex parte Kenichi Miyazaki, the U.S. Patent and Trademark Office Board of Patent Appeals and Interferences (USPTO BPAI) has decided that if a claim term is not used or defined in the specification, and the meaning of the term is not discernable, the claim is indefinite. Moreover, any inconsistency between a claim term and the patent specification may make the claim indefinite. Indeed, the presence of a claim term susceptible of two or more plausible constructions, renders the claim ambiguous and, hence, indefinite.
THE LESSON TO BE LEARNED: Don’t use claim terms that were not used, and clearly defined, in the specification.