"Amici curiae" means "friend of the court."  Rules of appellate procedure typically afford an opportunity, with the court's advance permission, for persons or entities other than the parties to the litigation having an interest in the outcome of the litigation to submit briefs advocating a position.  Typically, a court permits the submission of such briefs in cases presenting issues of significant economic, social, or political impact.

     Rule 29 of the Federal Rule of Appellate Procedure is an example.  It reads, in part: 

Rule 29. Brief of an Amicus Curiae

(a) When Permitted.

The United States or its officer or agency, or a State, Territory, Commonwealth, or the District of Columbia may file an amicus-curiae brief without the consent of the parties or leave of court. Any other amicus curiae may file a brief only by leave of court or if the brief states that all parties have consented to its filing.

(b) Motion for Leave to File.

The motion must be accompanied by the proposed brief and state:
(1) the movant's interest; and
(2) the reason why an amicus brief is desirable and why the matters asserted are relevant to the disposition of the case.

     The economic impact of the decision in ProCD was obvious.  Four amici curiae briefs were submitted on behalf of a number of parties:  Information Industry Association, American Medical Association, Association of American Publishers, Business Software Alliance, Software Publishers Association, American Committee for Interoperable Systems.