It is a legal defense in equity. P has brought a claim against D. But P waited so long to make the claim that P's harm was bigger than it should have been. D can assert that the bigger claim should be barred by laches. Let's say I dent your car, a tiny dent that could easily be touched up with a spot of paint. But you wait until the moisture gets in there and rusts the entire door. Now you sue me for the cost of repairing the entire door. I can raise the defense that your claim is barred by laches. You had a duty to mitigate damages when the dent occurred. You can still file a claim for the original, smaller repair cost, assuming that the statute of limitations has not run out.

Laches is an equitable defense similar to the statute of limitations (SOL). However, unlike the SOLs, it is not a fixed time set by law. Rather, it is a time that is determined by equity. I real life example would explain this best. Suppose you have the copyright to a building design. Suppose that you learn that another company is infringing on your copyright and building a structure based on your design. Now suppose that instead of suing right away, you wait two years and let them complete the building, and then you decide to sue for infringement. Here, you are in the SOL because you sued under the 3 year mark. However, your claim may be barred by laches because you chose to wait until after the company spent invested quite a bit of money in the project. Note: Laches is an equitable defense. Thus, in the above example, it would bar the copyright owner from suing for equitable relief (e.g. the destruction of the building), but not for legal damages (i.e. money). The key to laches is fairness. For example, laches will also be allowed if someone decides to wait and sue until after a known or planned destruction of evidence.