Douglas Harper's Etymology Dictionary
1610s, Latin, literally "friend of the court;" plural is amici curiae. From Latin amicus "friend," related to amare "to love" (see Amy) + curia "court" (see curia).
Wiktionary
n. (context legal English) a person/entity who has been allowed by the court to plead or make submissions but who, however, is not directly involved in the action.
WordNet
n. an adviser to the court on some matter of law who is not a party to the case; usually someone who wants to influence the outcome of a lawsuit involving matters of wide public interest [syn: friend of the court]
Wikipedia
An amicus curiae (literally, friend of the court; plural, amici curiae) is someone who although not a party to a case and unsolicited by any of the parties in the case, to assist the court in the case offers 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.
Usage examples of "amicus curiae".
After all, the authorities can always do as the Scottish Lord of Justiciary did in the Weinstein case: bring in a watchdog operant as an amicus curiae to be on the lookout for mental hanky-panky.
I kept a fairly complete record of the cases in which Vance participated as a kind of amicus curiae, little thinking that I would ever be privileged to make them public.
I have assumed that you are in some way an amicus curiae -- a friend of the court -- but if you are going to advise and interfere I want to know why, and who you are.