X factored

X is a procedural defense (and, in many countries such as the United States, Canada, Mexico and India, a constitutional right) that forbids a defendant from being tried twice for the same crime on the same set of facts.If the defendant succesfully proves that he has been acquitted or convicted of the same offense then the projected trial has to be prevented from proceeding.X has been used as a plot many a times in movies as well as in fiction , notably crime thrillers.A hollywood movie of the same name was released in 1999 starring Ashley Judd.Identify X?