The power of the jury to ignore a law and to return a verdict according to its conscience in a criminal case is known as

The power of the jury to ignore a law and to return a verdict according to its conscience in a criminal case is known as jury nullification.
Jury nullification is a discretionary act used when the jury want to "send a message" about some social issue.
Another scenario is when a jury feels the application of the law is unfair, unjust or is immoral in some way. want to "send a message" about some social issue.