In its strictest sense, jury nullification occurs when a jury returns a Not Guilty verdict even though jurors believe the defendant has broken the law. Because the Not Guilty verdict cannot be overturned, and because the jurors cannot be punished for their verdict, the law is said to be nullified in that particular case.
In what can be said to be a milder form of jury nullification, some of the jurors, or even just one in most cases, can hang the jury by maintaining a Not Guilty verdict even though they believe the defendant broke the law. There is no requirement that jurors must come to a unanimous verdict. If the jury cannot unanimously agree on a verdict of either Guilty or Not Guilty, this is known as a hung jury. When further deliberation clearly will be unproductive, the judge will declare a mistrial. The prosecution may or may not retry the case in the future, but the law has at least been nullified in the trial at hand.
Former prosecutor and current Georgetown University Law Center professor Paul Butler has dubbed another variation on this theme to be “jury nullification 2.0”. He used this term in reference to the case of Touray Cornell, a Missoula, Montana man charged with possession of 1/16th of an ounce of marijuana in a county that had passed a citizen initiative instructing law enforcement to make marijuana enforcement their lowest priority. Of 27 potential jurors questioned during voir dire, only five said they would vote to convict a person of possession of such a small amount of marijuana. Skeptical that it would even be possible to seat a jury, the judge in the case called a recess during which time the lawyers worked out a deal known as an “Alford plea” in which the defendant didn’t admit guilt.
When these kinds of rejections of enforcement of laws stack up over time, the laws become unenforceable. We’ve seen this rejection of the Fugitive Slave Laws and alcohol prohibition, for example, undermine such laws’ enforcement. Eventually it is no longer worth the time or hassle or embarrassment for government officials to try to enforce these laws. They may be further nullified in a sense either remaining on the books but not being enforced, or being repealed altogether
Other terms you may hear in place of jury nullification are conscientious acquittal, juror veto, or jury pardon.