Definition from Nolo’s Plain-English Law Dictionary
A rule from the Fifth Amendment to the U S Constitution that prohibits a criminal efendant from being twice made to stand trial for the same offense. A defendant is put "in jeopardy" once the jury is sworn. If the prosecutor moves to dismiss the case after that, the defendant cannot be retried. When a judge dismisses a case, however, a retrial is generally possible unless the dismissal was engineered by the prosecutor's misconduct, or there was no overriding necessity to dismiss the case. Double jeopardy protects defendants only for retrials brought within the original jurisdiction, which is why a defendant can be tried in federal court after being tried in state court. Double jeopardy does not prevent trial in a civil court on underlying facts that previously formed the basis of a criminal trial.
A is accused the theft of a watch from a store. He is arrested, charged and tried, but the jury acquits him. Any attempt to charge him again with the same crime would be a violation of double jeopardy, and unconstitutional.