Jury nullification is the legal power of a jury to declare a defendant "not guilty" even though the law would require a guilty verdict. In so doing, the jury "nullifies" the effect of the law in a particular case.
This doctrine was established in the trial of William Penn in England. Penn had joined the Quakers in London, a religion disliked by the King. In 1670, Penn held a worship service and was arrested allegedly for disturbing the King's peace At trial, the jurors heard testimony and the judge, as is still the custom, delivered jury instructions prior to its deliberations. The jury instructions included an order to find the Penn guilty.
But the jury refused to find Penn guilty. The judge angrily sent them back to continue deliberations. The jury returned again with its same verdict. The judge demanded "a verdict that the court will accept, and you shall be locked up without meat, drink, fire, and tobacco .... We will have a verdict by the help of God or you will starve for it." The jury went out three more times, and returned with the same verdict each time. Then it refused to deliberate any more and the judge fined and imprisoned them. Penn was also fined and imprisoned on a scurrilous new charge invented at trial (for donning a hat in the courtroom).
On appeal, the jurors won their independence and were released from jail, though that did not help Penn.
This case established the principle of "jury nullification," whereby a jury’s decision is final and dispositive even if it rejects the law. Jury nullification has been used frequently in America ever since, as in acquitting John Peter Zenger and acquitting defendants accused of violating the Alien and Sedition Act and the Fugitive Slave Act.