is the equitable
analog to the statute of limitations
and describes a defense of unfairness (when there is no other time limitation) that someone has waited too long before bringing a lawsuit. A successful defense of laches will find the court dismissing the request for equitable relief. However, the party may still have an action at law if the statute of limitations has not run.
The period afterwhich one had waited too long was unofficially set at six years, until undermined by the Federal Appeals Court.
A good example is the case of British Telecom's attempt to reign in fees for the use of hypertext. Professor Richard Stern discusses the notion in the context of BT  (pdf).