Jury nullification is the practice by which a jury returns a "not guilty" verdict despite believing the defendant technically violated the law, either because they consider the law unjust or its application to this defendant unjust. The Double Jeopardy Clause makes acquittals essentially final, so juries have unreviewable power to acquit on any basis.
Courts have long held that while juries have the power to nullify, they have no right to be told they have that power. Most jurisdictions instruct jurors that they "must follow the law as given," and judges typically dismiss prospective jurors who say they would consider nullification. New Hampshire is unusual in having a statute permitting nullification arguments.
Historical examples include Northern juries refusing to convict under the Fugitive Slave Act, Prohibition-era juries refusing to convict for alcohol offenses, and modern juries declining to convict in some drug-possession cases or against protesters.