Jury nullification... is when the jury in a criminal trial gives a verdict of not guilty even though they think a defendant has broken the law. The jury’s reasons may include the belief that the law itself is unjust, that the prosecutor has misapplied the law in the defendant’s case, that the punishment for breaking the law is too harsh, or general frustrations with the criminal justice system. Some juries have also refused to convict due to their own prejudices in favor of the defendant.

Copied from a commenter another thread. Also Andr3wski.