Exclusionary rule - principle that evidence cannot be used against a person if it was obtained illegally. This principle was established by the Supreme Court in the 1967 case, Mapp v. Ohio. In Nix v. Williams (1984), the Supreme Court ruled that evidence that had been illegally obtained could be used against someone in court if the prosecution could prove that the evidence "ultimately or inevitably would have been discovered by lawful means." In the same year, in United States v. Leon, the Court again restricted the exclusionary rule. The Court decided that, "when an officer acting with objective good faith has obtained a search warrant," the evidence obtained should be admissible in court, even if the warrant later proves to be faulty.