Doctrine of equivalents
The rule that you can infringe a patent without matching the literal words of its claims, as long as your version does effectively the same thing.
Without it, every patent claim could be dodged by swapping a trivial word or component. In Graver Tank (1950) the US Supreme Court framed the test still quoted today: does the substitute perform substantially the same function, in substantially the same way, to achieve substantially the same result? Europe handles the same question through the Protocol on the Interpretation of Article 69 EPC.
The doctrine has limits. Ground surrendered during prosecution can't be reclaimed through equivalence (Festo, 2002), and a claim can't be stretched over known prior art. In software, where the same algorithm can be written in endless ways, this is often where the fight is actually decided.