a legal Latin phrase, literally translated as "friend of the court," that refers to a person or entity that is not a party to a case that volunteers to offer information on a point of law or some other aspect of the case to assist the court in deciding a matter before it

cf. other permutations:

for an infant: acting as a next friend as claimant or guardian ad litem defending.
also for a medical or pychatriatric patient, as a litigation friend

This is the explanation given in Black's Law Dictionary 8th, since the types of intervention provided for in the Federal Rules of Civil Procedure (Rule 24) and in state courts ("intervention as of right" and "permissive intervention") do not coincide with the Spanish categories. "Intervención" is not an amicus curiae pleading (which is unknown in Spanish civil proceedings), since the intervenor ("interviniente") does indeed become a party to the proceedings, joining either the plaintiff or defendant, or taking a position adverse to both, rather than just present an opinion that he asks be considered by the court.

See Articles 13 and 14 of the Ley de Enjuiciamiento Civil (1/2000)
http://www.juridicas.com/base_datos/Privado/l1-2000.l1t1.htm...



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Note added at 1 day17 hrs (2006-05-24 08:52:17 GMT)
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For some reason, the program won't allow me to respond to Tom's comment, so I will do so here. Obviously, the "defensor judicial" is not exactly the same as a McKenzie Friend (we are comparing two different systems, and no equivalents actually exist), but it is perhaps the closest equivalent. But regardless of the relationship defensor judicial-McKenzie Friend, the role of a McKenzie Friend does not correspond to the definition of "intervención". The interviniente enters a proceeding to protect his own interests, not those of one of the litigants (the interviniente "tiene interés directo y legítimo en el resultado del pleito").