Lat. "friend of the court." Refers to a party that is allowed to provide information (usually in the form of a legal brief) to a court even though the party is not directly involved in the case at hand.
One, who as a stander-by, when a judge is doubtful or mistaken in a matter of law, may inform the court. And any one, as amicus curia, may make an application to the court in favor of an infant, though he be no relation.
A legal argument filed in a lawsuit by a person or organization not a party to the case, but who has an interest in the outcome. For example, in the Supreme Court abortion case, Webster v. Reproductive Services, amicus curiae briefs were filed by hundreds of pro-choice and anti-abortion organizations. The court may give the arguments in the amicus curiae brief as much or as little weight as it chooses.