Final answer:
The exclusionary rule prohibits the use of illegally obtained evidence in a criminal trial.
Step-by-step explanation:
The exclusionary rule states that any evidence obtained illegally, such as through an illegal search or seizure, cannot be used in a criminal trial. It was established in the Supreme Court cases Weeks v. United States (1914) and Mapp v. Ohio (1961). The rule not only applies to the directly obtained evidence, but also to any evidence developed as a result of the illegal search or seizure.