Jury nullification is the term for when a jury declines to convict a defendant despite overwhelming evidence of guilt. This can be a form of civil disobedience, a political statement against a specific law, or a show of empathy and support to the defendant.
“It’s not a legal defense sanctioned under the law,” said Cheryl Bader, associate professor of law at Fordham School of Law. “It’s a reaction by the jury to a legal result that they feel would be so unjust or morally wrong that they refuse to impose it, despite what the law says.”
If all vote not guilty it’s called an acquittal. If that happens, despite overwhelming evidence of guilt, it is jury nullification ending in an acquittal. If a mistrial happens, meaning the jury cannot reach a verdict, despite overwhelming evidence of guilt, it is jury nullification ending in a mistrial.
Source: I have been a public defender in NYC for 15 years.