(Latin: “friend of the court”) One who assists a court by furnishing information or advice regarding questions of law or fact. A person (or other entity, such as a state government) who is not a party to a particular lawsuit but nevertheless has a strong interest in it may be allowed, by leave of the court, to file an amicus curiae brief, a statement of particular views on the subject matter of the lawsuit. Such briefs are often filed in cases involving public-interest matters (e.g., enh1ment programs, consumer protection, civil rights).
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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:
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, the Electronic Frontier Foundation, the American Center for Law and Justice or NORML 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 litigants may wish to have their concerns heard. In the United States, federal courts often hear cases involving the constitutionality of state laws. In this case, 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.