A section 2(f) claim is an argument made to the U.S. Patent and Trademark Office (USPTO) that a mark has become distinctive.  This claim can be made by proving continuous and exclusive use of a mark for five years, or by producing other proof of secondary meaning.

Section 2(f) of the Lanham Act (15 U.S.C. 1052(f)) provides that marks that would otherwise be denied registration, can be registered if they have become distinctive by acquiring secondary meaning.