Amicus curiae
Defined in 3 dictionaries — Ballentine's (1916), Black's (1891), Bouvier (1839)
Ballentine's Law Dictionary
James A. Ballentine · 1916
A Dictionary of Law
Henry Campbell Black · 1891
Lat. A friend of the court. A by-stander (usually a counsellor) who interposes and volunteers information upon some matter of law in regard to which the judge is doubtful or mistaken, or upon a matter of which the court may take judicial cognizance. When a judge is doubtful or mistaken in matter of law, a by-stander may inform the court thereof as amicus curiae. Counsel in court frequently act in this capacity when they happen to be in possession of a case which the judge has not seen, or does not at the moment remember. Holthouse. It is also applied to persons who have no right to appear in a suit, but are allowed to introduce evidence to protect their own interests. 11 Tex. 699, 701, 702.
A Law Dictionary, Adapted to the Constitution and Laws of the United States
John Bouvier · 1839
a friend of the court, in practice. 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 curie, may make an application to the court in favour of an infant, though he be no relation, 1 Ves. Sen. 318.