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Amicus curiae or amicus curiĆ¦ (plural amici curiae) is a legal Latin phrase, literally translated as "friend of the court", that refers to someone, not a party to a case, who volunteers to offer information on a point of law or some other aspect of the case to assist the court in deciding a matter before it. The information may be a legal opinion in the form of a brief - testimony that has not been solicited by any of the parties - or a learned treatise on a matter that bears on the case. The decision whether to admit the information lies with the discretion of the court.
The role of an amicus is often confused with that of an intervener (see Intervention (law)). The role of an amicus is as stated by Salmon LJ (as Lord Salmon then was) in Allen v Sir Alfred McAlpine & Sons Ltd  2 QB 229 at p.266 F-G where he said:
I had always understood that the role of an amicus curiae was to help the court by expounding the law impartially, or if one of the parties were unrepresented, by advancing the legal arguments on his behalf.
The situation most often noted in the press is when an advocacy group files a brief in a case before an appellate court to which it is not a litigant. Appellate cases are normally limited to the factual record and arguments coming from the lower court case under appeal; attorneys focus on the facts and arguments most favorable to their clients. Where a case may have broader implications, amicus curiae briefs are a way to introduce those concerns, so that the possibly broad legal effects of court decisions will not depend solely on the parties directly involved in the case.
In prominent cases, amici curiae are generally organizations with sizable legal budgets. Non-profit legal advocacy organizations such as the American Civil Liberties Union or the Electronic Frontier Foundation or the American Center for Law and Justice frequently submit such briefs to advocate for or against a particular legal change or interpretation. If a decision could affect an entire industry, companies other than the litigant(s) may wish to have their concerns heard. In the United States, Federal courts often hear cases involving the constitutionality of state laws: other states may file briefs as amici curiae when their laws are likely to be affected.
Amici curiae that do not file briefs often present an academic perspective on the case. For example, if the law gives deference to a history of legislation of a certain topic, a historian may choose to evaluate the claim using their expertise. An economist, statistician, or sociologist may choose to do the same.
The court has broad discretion to grant or to deny permission to act as amicus curiae. Generally, cases that are very controversial will attract a number of such briefs.