In the United Kingdom and the Commonwealth realms, the term is to turn Queen's or King's evidence, depending on the sex of the reigning monarch. The term "turning approver" or "turn king's approver" was also historically used; an approver "not only admitted his own guilt to a crime but also incriminated his accomplices both past and present" in exchange for avoiding a death sentence (and obtaining a lesser penalty, such as life imprisonment or abjuration of the realm) or improving prison conditions.
In American parlance, a defendant who agrees to cooperate with prosecutors and give information against co-conspirators (often those with greater culpability) is also said to flip.
Witnesses who have turned state's evidence have been important in organized crime cases in the United States, such as those against the American Mafia. The first mafiosi who turned state's evidence, such as Joseph Valachi and Jimmy Fratianno, did so in response to threats on their life from Mafia associates; later cooperators were motivated to cooperate in order to avoid heavy sentences, such as those provided for under the RICO Act. Some who turned state's evidence were permitted to participate in the Witness Security Program (WITSEC). Among the highest-ranking Mafia members to ever turn state's evidence was Salvatore Gravano ("Sammy the Bull"), an underboss of the Gambino crime family who pleaded guilty to 19 murders and agreed to testify against family boss John Gotti. (Gravano was sentenced to 20 years; Gotti was sentenced to life imprisonment in 1992). Joseph Massino was also the first boss of one of the Five Families in New York City to turn state's evidence.
The incentives to turn state's evidence, or to not to do so, are explored in the famous prisoner's dilemma, created by Merrill Flood and Melvin Dresher.