jury nullification

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Refusal of a jury to find a defendant guilty, even when the facts establish guilt, because of a conclusion that conviction would be contrary to some other concept of justice.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun law An acquittal by a jury of a defendant ignoring the facts of the case and/or the law.