King's evidence

To turn state's evidence is for an accused or convicted criminal to testify as a witness for the state against his associates or accomplices. Turning state's evidence is occasionally a result of a change of heart or feelings of guilt, but is more often in response to a generous offer from the prosecution, such as a reduced sentence or a favorable location for serving time. In particularly high-profile or dangerous cases such as organized crime trials and other federal crimes trials, witnesses may be offered immunity from prosecution even if they have committed serious crimes themselves, sometimes even murder. The state may also offer the witness a place in the witness protection program, giving them a new identity, so they need not fear retaliation from their former accomplices.

In the United Kingdom and the Commonwealth realms, the term is to turn Queen's or King's evidence, depending on the reigning monarch's sex.

In India (and historically in the British Isles) the term turning approver is used for someone giving full disclosure about a crime in return for pardon under Sections 306 to 308 of the Code of Criminal Procedure, 1973, though the term is not used in the Code itself.

...
Wikipedia