In jurisprudence, double jeopardy is a procedural defense (and, in the US, a constitutional right), which forbids a defendant from being tried a second time for a crime, after having already been tried for the same crime.
The phrase "double jeopardy" stems from the Fifth Amendment to the US Constitution: "nor shall any person be subject for the same offence to be twice put in jeopardy of life or limb." This clause is intended to limit prosecutorial abuse by the government in repeated prosecution, for the same offense, as a means of harassment or oppression.
This law is occasionally referred to as a legal technicality, especially when it is used as a criminal defense. It is not uncommon, for example, for police to uncover new evidence proving the guilt of someone previously acquitted of a serious crime. There is little they can do in this case, because the first acquittal is final, and the defendant may not be tried again despite the new evidence.
Some conditions that might be seen as double jeopardy are not considered so by the courts. For example, a re-trial after a mistrial does not violate the double-jeopardy clause (the legal explanation is that the defendant waives that objection by moving for a mistrial, and the court merely grants that motion). Thus, mistrials due to hung juries do not attach jeopardy and can be retried, but cases which have been dismissed constitute a trial for these purposes.
Double jeopardy is not implicated for separate offenses or in separate jurisdictions arising from the same act. For example, in US v. Felix (1992), the Supreme Court ruled: "a[n]...offense and a conspiracy to commit that offense are not the same offense for double jeopardy purposes." As another example, a state might try a defendant for murder, after which the federal government might try the same defendant for a federal crime (perhaps a civil rights violation or kidnapping) related to the same act. For example, the policemen who beat up black motorist Rodney King in 1991 were acquitted by a county court of the accusation of assault; some where sentenced in federal court for violating his civil rights. Similar techniques were used for prosecuting racially-motivated crimes in the Southern United States in the 1960s, which were not actively prosecuted nor convicted in local courts. Another example, Timothy McVeigh, was sentenced to death for murdering eight US federal employees with a bomb, but could also have been tried in state court for murdering the rest of those whom he killed in the same explosion.
Acquittal in a criminal case does not prevent the defendent from being the defendent in a civil suit relating to the same incident.
In US v. Aleman, a man who bribed a judge was tried again for murder after having been acquitted. The Supreme Court ruled that his first trial did not really put him in jeopardy because of his bribe.
British Home Secretary David Blunkett in 2003 announced plans to restrict or abolish the double jeopardy rule in the United Kingdom. The full nature of the change has yet to be put into legislative form.