state's evidence

evidence given by an accomplice in a crime who becomes a voluntary witness against the other defendants: The defendants' case was lost when one of them turned state's evidence.

evidence for the state, especially in criminal trials.

the evidence for the prosecution given on behalf of a state in a criminal prosecution

evidence given for the state by an accomplice against his former associates in crime (esp in the phrase turn state's evidence): Brit equivalent: king's evidence, queen's evidence