(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., entitlement programs, consumer protection, civil rights).