doctrine of equivalence

invoked when a patent's claim is not literally infringed, but (here comes the is the legal jargon) "the infringing device uses substantially the same means in substantially the same way to get substantially the same result". This is a gray area without hard-and-fast rules, designed to prevent competitors from making trivial changes to an invention to escape infringement liability. Equivalence is determined by either comparing the claimed and accused devices as wholes, or by going through the claims element-by-element, depending on the circumstances and the opinions of the court. The doctrine of equivalence tends to favor the patentee, but this tendency is balanced out by use of prosecution history estoppel.