Formerly a U.S. equitable defense based on prolonged acquiescence by the patent (or copyright) holder to the infringement, which precluded the collection of damages for past infringement – the application of laches in US patent cases was effectively abolished by the 2017 US Supreme Court decision in SCA Hygiene Products AB v. First Quality Baby Products AB – which applied the same analysis that the Supreme Court had used with respect to Copyright, eliminating the defence in Petrella v. Metro-Goldwyn-Mayer.
Under the old rule the infringer did not need to show that it knew of the patent for laches to apply, but rather that the patent holder’s delay was unreasonable and that the infringer’s position changed adversely due to such delay – this adverse change is often referred to as “prejudice.” In US law the burden of proof with respect to such irreparable harm resulting from delay lies with the accused infringer for the first six years of acquiescence, at which point it shifts to the patent holder. For mysterious reasons, the word laches is often italicised.
SCA Hygiene specifically did not address or abolish the similar but different defence of equitable estoppel.