Turn state’s evidence (also known as “turn King’s evidence”) or to "flip” means the defendant chose to reveal valuable evidence to the prosecutor, in exchange for a reduction of the charge or the dismissal of some charges. When the defendant “flips” they are said to have "turned state's evidence.”
This is common in instances of organized crime when a defendant provides information on a co-defendant or other members' crimes. The defendant tries to flip for a better sentence, but it should be agreed by the judge. The right to flip is a constitutional right; see Brady v. United States, but it should be made voluntarily and follow the Federal Rules of Criminal Procedure Rule 11.
[Last updated in April of 2022 by the Wex Definitions Team]