Jury Nullification

A jury's knowing and deliberate rejection of the evidence or refusal to apply the law either because the jury wants to send a message about some social issue that is larger than the case itself, or because the result dictated by law is contrary to the jury's sense of justice, morality, or fairness. 

Jury nullification is a discretionary act, and is not a legally sanctioned function of the jury.  It is considered to be inconsistent with the jury's duty to return a verdict based solely on the law and the facts of the case.  The jury does not have a right to nulification, and counsel is not permitted to present the concept of jury nullification to the jury.  However, jury verdicts of acquittal are unassailable even where the verdict is inconsistent with the weight of the evidence and instruction of the law.

See U.S. v. Thomas, 116 F.3d 606 (2d Cir. 1997).