WordNet (r) 3.0 (2006):

amicus curiae
    n 1: 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: amicus curiae, friend of the court]

Moby Thesaurus II by Grady Ward, 1.0:

74 Moby Thesaurus words for "amicus curiae":
   JA, advocate, agent, alter ego, alternate, assessor, attorney,
   attorney-at-law, backup, backup man, barmaster, barrister,
   barrister-at-law, champion, chancellor, circuit judge, counsel,
   counselor, counselor-at-law, deputy, dummy, executive officer,
   exponent, figurehead, friend at court, intercessor, judge advocate,
   judge ordinary, jurat, justice in eyre, justice of assize, lawyer,
   lay judge, legal adviser, legal assessor, legal counselor,
   legal expert, legal practitioner, legalist, lieutenant, locum,
   locum tenens, master, military judge, mouthpiece, ombudsman,
   ordinary, paranymph, pinch hitter, pleader, police judge,
   presiding judge, probate judge, proctor, procurator, proxy,
   puisne judge, recorder, representative, sea lawyer,
   second in command, secondary, self-styled lawyer, solicitor,
   stand-in, substitute, surrogate, understudy, utility man, vicar,
   vicar general, vice, vice-chancellor, vicegerent



Bouvier's Law Dictionary, Revised 6th Ed (1856):

AMICUS CURIAE, practice. A friend of the court. 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 curia, may make
an application to the court in favor of an infant, though he be no relation.
1 Ves. Sen. 313.