Metaphor used to describe a doctrine of evidence law, whereby evidence obtained in a criminal investigation without a warrant, or with a defective warrant, cannot be used in the criminal trial.
Since at least Weeks v. United States, 232 U.S. 383 (1914), evidence obtained in violation of rights protected by the Fourth Amendment to the Federal Constitution must be excluded in federal criminal prosecutions. The Fourth Amendment prohibits unreasonable searches and seizures. This doctrine is called variously the “exclusionary rule” or the “fruit of the poisoned tree” doctrine.
The rationale for the exclusionary rule is that it is more important for society to deter government abuse of power than it is to get a conviction in a particular criminal case. There are numerous “exceptions”, or perhaps distinctions to be made. The biggest loophole is the “inevitable discovery” exception: if the evidence would have been obtained anyway, with or without the illegal search, then the evidence is not really the “fruit” of the illegal search and may be used at trial.
One way to see the wisdom in the exclusionary rule to consider the alternative. The alternative of simply not enforcing the Fourth Amendment is not seriously considered. One could make law enforcement officers liable for illegal searches, either as an obstruction of justice crime or a civil rights case for damages. The courts, however, have seen fit to grant immunity, or at least qualified immunity, to insulate law officers from prosecution and lawsuits for all but the most egregious abuses. Instead, the evidence seized is excluded, and the police are encouraged to use proper procedure, without fear that the police officers themselves might be arrested or sued if they make a mistake.
The Supreme Court's recent decision to leave the Miranda rule intact shows that this general framework is not likely to be rejected anytime soon.
Battles between conservatives and liberals on search and seizure law have moved away from the general exclusionary rule itself, on to whether particular types of searches or seizures are "reasonable" under the Fourth Amendment, e.g. drug tests, roadblocks, whether particular facts constitute "probable cause" or "reasonable suspicion" for a search.