An equitable defense in United States patent cases, which precludes enforcement of a patent against an infringer who was aware of the patent but who relied on the prolonged inaction of the patent-holder to conclude that the patent-holder would not enforce the patent and has detrimentally changed his or her position based on that reliance, i.e., the putative defendant has suffered irreparable harm, often described as “prejudice.”
Prejudice giving rise to equitable estoppel usually has to be proved by the alleged infringer, but after six years of inaction it becomes presumptive and must be disproved by the patent-holder (See A.C. Aukerman Co. v. R.L. Chaides Constr. Co. (Federal Circuit 1992, en banc). However, six years is not essential and courts have frequently found equitable estoppel arising after shorter period of inaction. Equitable estoppel should be compared with the similar, but now severely curtailed defense of laches. Equitable estoppel is, in principle, a different legal concept from collateral estoppel.