What is Amicus Curiae?

Legal Definition
An amicus curiae (literally, friend of the court; plural, amici curiae) is someone who is not a party to a case and is not solicited by a party, but who assists a court by offering information that bears on the case. The decision on whether to admit the information lies at the discretion of the court. The phrase amicus curiae is legal Latin.
-- Wikipedia
Legal Definition
Latin for "friend of the court." Frequently, a person or group who is not a party to a lawsuit, but has a strong interest in the matter, will petition the court for permission to submit a brief in the action with the intent of influencing the court's decision. See, e.g. American Airlines v. Wolens, 513 US 219 (1995).
Legal Definition
A party that is not involved in litigation but gives expert testimony when the court asks. They can support public interest not being addressed in the trial.
Legal Definition
Practice. A friend of the court. One, who as a stander by, when a judge is doubtful or mistaken in a matter of law, may inform the court. 2 Inst. 178; 2 Vin. Abr. 475; and any one, as amicus curia, may make an application to the court in favor of an infant, though he be no relation. 1 Ves. Sen. 313. AMITA. A paternal aunt; the sister of one's father. Inst. 3, 6, 3.
-- Bouviers Law Dictionary
Legal Definition
A friend of the court; one who volunteers assistance to the court on a matter of law. See 46 Am. St. Rep. 45.
-- Ballentine's Law Dictionary