The terms moot, mootness and moot point are used both in English and in American law, although with significantly different meanings.

In the legal system of the United States, 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 depriving the matter of practical significance or rendering it purely academic.

The U.S. development of this word stems from the practice of moot courts, in which hypothetical or fictional cases were argued as a part of legal education. These purely academic settings led the U.S. courts to describe cases where developing circumstances made any judgment ineffective as "moot".

The mootness doctrine can be compared to the ripeness doctrine, another court rule (rather than law) that holds that judges should not rule on cases based entirely on anticipated disputes or hypothetical facts. These rules and similar doctrines, taken together, prevent the federal courts of the United States from issuing advisory opinions, as required by the Case or Controversy Clause of the United States Constitution.

The usage in the British legal system, on the other hand, is that the term "moot" has the meaning of "remains open to debate" or "remains unresolved". The divergence in usage was first observed^[when?] in the United States, and the extent to which the U.S. definition is used in U.S. jurisprudence and public discourse has ensured it is rarely used in a British courtroom. This is partially to avoid ambiguity, but also because the British definition is rarely relevant in practical cases.

In the U.S. federal judicial system, a moot case must be dismissed, there being a constitutional limitation on the jurisdiction of the federal courts. 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 ordinarily beyond the power of the court to decide, provided it does not fall within one of the recognized exceptions.

A textbook example of such a case is the United States Supreme Court case DeFunis v. Odegaard, 416 U.S. 312 (1974). The plaintiff was a student who had been denied admission to law school, and had then been provisionally admitted during the pendency of the case. Because the student was slated to graduate within a few months at the time the decision was rendered, and there was no action the law school could take to prevent that, the Court determined that a decision on its part would have no effect on the student's rights. Therefore, the case was dismissed as moot.

However, there is disagreement as to both the source of the standards, and their application in the courts. Some courts and observers opine that cases must be dismissed because this is a constitutional bar, and there is no "case or controversy"; others have rejected the pure constitutional approach and adopted a so-called "prudential" view, where dismissal may depend upon a host of factors, whether the particular person has lost a viable interest in the case, or whether the issue itself survives outside the interests of the particular person, whether the circumstance are likely to recur, etc. In actual practice, the U.S. federal courts have been uneven in their decisions, which has led to the accusation that determinations are ad hoc and 'result-oriented.'