Nolo's Plain-English Law Dictionary

Turn State's Evidence

The decision by a person charged with a crime, or suspected of a crime, to cooperate with the prosecution and testify against another participant in the criminal activity. In exchange for giving information or testifying, the person who has "turned" gets some sort of lenient treatment, such as a reduced charge, a plea bargain to a lesser charge, or a promise that the prosecution will recommend a light sentence.