In criminal law, an acquittal is a verdict of not guilty, or some similar end of the proceeding that terminates it with prejudice without a verdict of guilty being entered against the accused. The opposite result is a conviction.
In the common law tradition, an acquittal formally certifies the innocence of the accused, as far as the criminal law is concerned. This is so even where the prosecution is abandoned nolle prosequi. Under the rules of double jeopardy and autrefois acquit, an acquittal operates to bar the retrial of the accused for the same offense, even if new evidence surfaces that further implicates the accused. The effect of an acquittal on criminal proceedings is the same whether it results from a jury verdict, or whether it results from the operation of some other rule that discharges the accused.
In England and Wales, which share a common legal system, the Criminal Justice Act 2003 creates an exception to the double jeopardy rule, by providing that retrials may be ordered if "new and compelling evidence" comes to light after an acquittal for a serious crime. Also the Criminal Procedure and Investigations Act 1996 permits a "tainted acquittal" to be set aside in circumstances where it is proved beyond reasonable doubt that an acquittal has been obtained by violence or threats of violence to a witness or juror.
In modern England and Wales, and in all countries that substantially follow English criminal procedure, an acquittal normally results in the immediate liberation of the defendant from custody, assuming no other charges against the defendant remain to be tried. However, until 1774 a defendant acquitted by an English or Welsh court would be remanded to jail until he had paid the jailer for the costs of his confinement. It was known for acquitted persons to die in jail for lack of jailer's fees.
With one exception, in the United States an acquittal cannot be appealed by the prosecution because of the prohibition against double jeopardy. The U.S. Supreme Court has ruled:
It was decided in Fong Foo v. United States, 369 U.S. 141 (1962) that a judgement of acquittal by a jury cannot be appealed by the prosecution. In United States v. Jenkins, 420 U.S. 358 (1975), this was held applicable to bench trials. In Arizona v. Rumsey, 467 U.S. 203 (1984), it was ruled that in a bench trial, when a judge was holding a separate hearing after the jury trial, to decide if the defendant should be sentenced to death or life imprisonment, the judge decided that the circumstances of the case did not permit death to be imposed. On appeal the judge's ruling was found to be erroneous. However, even though the decision to impose life instead of death was based on an erroneous interpretation of the law by the judge, the finding of life imprisonment in the original case constituted an acquittal of the death penalty and thus death could not be imposed upon a subsequent trial. Even though the acquittal of the death penalty was erroneous in that case, the acquittal must stand.
The only exception to an acquittal being final is if the defendant was never in jeopardy at all at trial. If a defendant bribes a judge and obtains acquittal as a result of a bench trial, the acquittal is not valid because the defendant was never in jeopardy in the first place. Harry Aleman v. Judges of the Criminal Division, Circuit Court of Cook County, Illinois, et al., 183 F.3d 302 (1998).
An acquittal, while conclusive as to the criminal law, does not necessarily bar private civil actions in tort or on some other grounds as a result of the facts alleged in the charge. For example, O.J. Simpson was held civilly liable for wrongful death even after being tried and acquitted of murder. In federal states it also does not bar prosecution for the same offences under a statute at a different level of government. For example, in the United States someone acquitted of a state murder charge can be retried for the same actions on a federal charge of violating civil rights.