prosecution history estoppel

the other side of the coin from doctrine of equivalence; in patent infringement lawsuits, it tends to favor the accused infringer. If, during the examination of a patent by the PTO in the course of the issuance process, the patentee had to change his or her claims to avoid existing prior art and avoid a rejection of his application as not novel, the pantentee is held to those new limitations for the rest of the life of the patent. The patentee can't go back later to the original, broader claims. Like the doctrine of equivalence, this is also not a hard-and-fast rule; in some cases, if the court determined that the restrictions made were unnecessarily broad, the patentee has been allowed to regain some ground that he or she might have given up to avoid prior art.

This is a confusing concept, so here's a silly example. Clyde files a patent claiming a table with four legs, a table with four-and-a-half legs, and a table with five legs, but the PTO finds that there is already a patent on a table with four legs. In order to avoid this prior art, Clyde cuts out the "four legs" and the "four-and-a-half legs" claims, and he gets his patent on the remaining claim of a table with five legs. Later on, Bonnie gets a patent on a table with four-and-a-half legs (because really the prior art is only on a four-legged table). Clyde can't sue Bonnie for infringement, even though he had claimed a four-and-a-half-legged table first, because he cut out that claim during his patent's appeal process.

See File Wrapper Estoppel.

This is synonymous with file wrapper estoppel.