A rule that provides that otherwise admissible evidence cannot be used in a criminal trial if it was the result of illegal police conduct WordNet 3.0 & Open English WordNet

In the United States, the exclusionary rule is a legal rule, based on constitutional law, that prevents evidence collected or analyzed in violation of the defendant's constitutional rights from being used in a court of law. Wikipedia

A US legal principle Wikipedia Disambiguation

A doctrine which requires that evidence obtained as the result of an illegal act on the part of law enforcement personnel (such as a warrantless search, or continued questioning a witness who has invoked the right of counsel) must therefore be excluded from being admitted as evidence in a trial. This rule does not apply in civil proceedings, although statutes sometime specifically provide for exclusion of such evidence. Wiktionary