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Laches is a doctrine in equity whereby courts can deny relief to a claimant with an otherwise valid claim when the party bringing the claim unreasonably delayed asserting the claim to the detriment of the opposing party. The doctrine is also commonly referred to as estoppel by laches

Importantly, laches does not apply merely due to the passage of time before bringing a legal claim. The justification for the doctrine is that the delay was unreasonable on the part of the plaintiff, and the changed conditions due to the delay render granting the relief sought inequitable. If the delay on the plaintiff’s part can be satisfactorily explained by some reason like lack of information, the delay may be excused. 

In the realm of intellectual property law, prosecution laches refers to an affirmative defense against patent infringement based on the patentee failing to enforce patent rights in a timely manner. 

[Last updated in June of 2023 by the Wex Definitions Team]