What is the "fruit of the poisonous tree" doctrine?

The "fruit of the poisonous tree" doctrine is most often associated with violations of the Fourth Amendment's prohibition of unreasonable searches and seizures. See Wong Sun v. United States, 371 U.S. 471 (1963). It prohibits at trial the use of evidence obtained directly or indirectly through an unlawful search or seizure. Id. at 484. In that context, the exclusionary rule is necessary to "make effective the fundamental constitutional guarantees of sanctity of the home and inviolability of the person." Id.