Final answer:
The exclusionary rule is a legal principle that prevents evidence gathered by unconstitutional means from being used against the accused in a trial.
Step-by-step explanation:
The legal principle you are referring to is called the exclusionary rule. It states that evidence gathered by unconstitutional means cannot ordinarily be used against the accused in a trial. This means that if the police conduct an illegal search or seizure without a warrant and find evidence of a crime, that evidence would generally be deemed inadmissible in court.
In the 1961 Supreme Court case Mapp v. Ohio, the court ruled that evidence obtained without a warrant, and not falling under any exceptions, cannot be used as evidence in a state criminal trial. This decision led to the widespread application of the exclusionary rule, which had originally been established at the federal level in 1914.
However, there are exceptions to the exclusionary rule. For example, evidence may still be used if the police believed the warrant they executed was valid or if the evidence would have been discovered anyway through legal means.