In law, a matter is moot if further legal proceedings with regard to it can have no effect, or events have placed it beyond the reach of the law. Thereby the matter has been deprived of practical significance or rendered purely academic. This is a little different from the ordinary meaning of moot, which means to raise an issue. The shift in usage was first observed in the United States.

In the U.S. federal judicial system, a moot case must be dismissed. The reason for this is that Article Three of the United States Constitution limits the jurisdiction of all federal courts to "cases and controversies". Thus, a civil action or appeal in which the court's decision will not affect the rights of the parties is beyond the power of the court to decide.

Occasionally a court will grant an exception to this mootness rule in cases relating to the public interest. For example, in the 1973 Supreme Court case of Roe v. Wade, challenging a Texas law forbidding abortion in most circumstances, the state argued that the case was moot because plaintiff Roe was no longer pregnant by the time the case was heard. The Court granted a hearing anyway, holding that the public interest was served by deciding the question even if circumstances made it impossible for Roe herself to benefit. The Court cited Southern Pacific Terminal Co. v. ICC, 219 U.S. 498, 515 (1911), which had held that a case was not moot when it presented an issue that was "capable of repetition, yet evading review". Perhaps in response to increasing workloads at all levels of the judiciary, the recent trend in the Supreme Court and other U.S. courts has been to construe this exception rather narrowly.

The U.S. state courts are not subject to the Article III limitations on their jurisdiction, and some state courts may render opinions in moot cases where the establishment of a legal precedent is desirable. For instance, in some state courts the prosecution can lodge an appeal after a defendant is acquitted: although the appeal court cannot set aside a not guilty verdict due to double jeopardy, it can issue a ruling as to whether a trial court's ruling on a particular issue during the trial was erroneous.