Amicus Curiae literally translated from Latin is "friend of the court." Plural is "amici curiae."
Generally, it is referencing a person or group who is not a party to an action, but has a strong interest in the matter. This person or group will petition the court for permission to submit a brief in the action intending to influence the court’s decision. Such briefs are called "amicus briefs."
Rule 37 of the Rules of the Supreme Court of the United States dictates the content, format, and circumstances of amicus briefs before the U.S. Supreme Court.
In general, Rule 29 of the Federal Rules of Appellate Procedure governs amici curiae in federal courts.
State rules of civil and appellate procedure govern amici curiae in state cases.
In addition to advocating for an outcome through briefs, amici curiae sometimes participate in oral arguments before an appellate court.
See Whole Woman's Health v. Hellerstedt for a recent example of amici curiae submitting briefs to the Court, and note how frequently the majority and concurring opinions cite to amicus briefs.
[Last updated in June of 2022 by the Wex Definitions Team]