hung jury

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

• noun A jury that is unable to reach a verdict because its members are unable to reach unanimous agreement (as in criminal cases) or the extent of agreement required by law (as in civil cases).

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

• noun A jury in a legal trial which is unable to reach a verdict because jurors are unable to agree.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a jury that is unable to agree on a verdict (the result is a mistrial)