Literally a “friend of the court,” this Latin term refers to third parties to litigation, particularly appellate litigation, who file briefs (memoranda) with the court, urging a particular outcome in the case, usually on public policy grounds. Amicus Curiæ/Curiae are very prevalent in U.S. appellate procedure and also in cases before the European Court of Justice or European Court of First Instance.
The effectiveness of such amicus briefs is hard to assess, though they may be helpful in obtaining reconsideration en banc or writs of certiorari. Amicii are not supposed to have any direct vested interest in the case, though in reality they are usually approached or encouraged by one party or the other to intervene in this way; if reviewing courts (who are not naïve) suspect that amicii are in fact partisans of one party or the other, they may not give much weight to their arguments or briefs.
The use of the grapheme æ or Ash is more common in European practice – it represents a diphthong common in Latin and Greek and is an accepted letter in several European languages, while in French æ is typically used in “loan words” imported directly from Latin and Greek.