What is jury nullification and what are its origins and history? What actions by a juror would be considered nullification?
Quoting from here,
Jury nullification occurs when a jury returns a verdict of "Not Guilty" despite its belief that the defendant is guilty of the violation charged. The jury in effect nullifies a law that it believes is either immoral or wrongly applied to the defendant whose fate they are charged with deciding.
In essence, a jury decides that a law should not be legal in the situation, and as such the charge is unwarranted.
One of the first cases was in the trial of John Peter Zenger, in 1735, where a law against libels was used against him, and subsequently nullified by a jury. It was subsequently used against the famous Alien and Sedition Acts, as well as The Fugitive Slave Laws.
Zenger was the first case in America; in 1670, it was used in the case of William Penn and William Mead, who were acquitted of "illegal assembly" as Quakers. In an interesting twist, the jurors were imprisoned, as jury nullification was not explicitly legal, but they were later released.
Interestingly enough, according to The New York Times
In 1895, the Supreme Court ruled that jurors had no right, during trials, to be told about nullification. The court did not say that jurors didn’t have the power, or that they couldn’t be told about it, but only that judges were not required to instruct them on it during a trial.
The Times also wrote that nullification had been used against laws against alcohol and gay marriage, though it did not cite specific cases.
A jury is charged with finding fact, and is supposed to take the law as given to them by a judge.
"Jury nullification" occurs when a jury bends the law to produce a desired result. This was made possible by English common law, which gave juries great latitude in determining what they would rule on. The underlying power may go all the way back to the Magna Carta of 1215.
One example occurred in "Dickensian" England where theft of an item value at one shilling (twelve pence) or greater carried the death penalty. In order to spare a young defendant this fate, the jury valued a diamond necklace at only 11 pence.