A doctrine which says that even if a patent claim does not literally read on a possibly infringing device, it can be read more broadly providing it does not read on the prior art. It is designed to allow the inventor to assert a patent where the differences between the inventor's and an infringer's product are not substantial.
One literally infringes a claim where every element of the claim is expressly satisfied by a device, process, or composition of matter. Under the judicially created doctrine of equivalents, one may be held liable as an infringer even if one does not literally infringe a patent. In general, it is an equitable concept employed to prevent someone from getting the benefit of the invention by making a minor change that avoids literal infringement. See also the related concept of file wrapper estoppel.