The Collaborative International Dictionary of English v.0.48:

Intervention \In`ter*ven"tion\, n. [L. interventio an
   interposition: cf. F. intervention.]
   [1913 Webster]
   1. The act of intervening; interposition.
      [1913 Webster]

            Sound is shut out by the intervention of that lax
            membrane.                             --Holder.
      [1913 Webster]

   2. Any interference that may affect the interests of others;
      especially, of one or more states with the affairs of
      another; -- the intervention of one state in the affairs
      of another is typically unwelcome by the state being
      intervened in, but some cases of mediation between states
      may be called intervention. Opposed to nonintervention.
      [1913 Webster +PJC]

            Let us decide our quarrels at home, without the
            intervention, of any foreign power.   --Sir W.
                                                  Temple.
      [1913 Webster]

   3. (Civil Law) The act by which a third person, to protect
      his own interest, interposes and becomes a party to a suit
      pending between other parties.
      [1913 Webster]

WordNet (r) 3.0 (2006):

intervention
    n 1: the act of intervening (as to mediate a dispute, etc.); "it
         occurs without human intervention" [syn: intervention,
         intercession]
    2: a policy of intervening in the affairs of other countries
       [syn: intervention, interference] [ant:
       noninterference, nonintervention]
    3: the act or fact of interposing one thing between or among
       others [syn: interposition, intervention]
    4: (law) a proceeding that permits a person to enter into a
       lawsuit already in progress; admission of person not an
       original party to the suit so that person can protect some
       right or interest that is allegedly affected by the
       proceedings; "the purpose of intervention is to prevent
       unnecessary duplication of lawsuits"
    5: care provided to improve a situation (especially medical
       procedures or applications that are intended to relieve
       illness or injury) [syn: treatment, intervention]

Moby Thesaurus II by Grady Ward, 1.0:

37 Moby Thesaurus words for "intervention":
   encroachment, entrance, entrenchment, impingement, imposition,
   incursion, infiltration, influx, infringement, injection, inroad,
   insinuation, interagency, intercession, intercurrence,
   interference, interjacence, interjection, interlocation,
   interloping, intermediacy, intermediation, interposition,
   interposure, interruption, intervenience, intrusion, invasion,
   involvement, irruption, mediation, obtrusion, sandwiching,
   stepping in, trespass, trespassing, unlawful entry



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

INTERVENTION, civil law. The act by which a third party becomes a party in a
suit pending between other persons.
     2. The intervention is made either to be joined to the plaintiff, and
to claim the same thing he does, or some other thing connected with it or,
to join the defendant, and with him to oppose the claim of the plaintiff,
which it is his interest to defeat. Poth. Proced. Civ. lere part. ch. 2, s.
6, Sec. 3. In the English ecclesiastical courts, the same term is used in
the same sense.
     3. When a third person, not originally a party to the suit or
proceeding, but claiming an interest in the subject-matter in dispute, may,
in order the better to protect such interest, interpose his claim, which
proceeding is termed intervention. 2 Chit. Pr. 492; 3 Chit. Com. Law, 633; 2
Hagg. Cons. R. 137; 3 Phillim. R. 586; 1 Addams, R. 5; Ought. tit. 14; 4
Hagg. Eccl. R. 67 Dual. Ad. Pr. 74. The intervener may come in at any stage
of the cause, and even after judgment, if an appeal can be allowed on such
judgment. 2 Hagg. Cons. R. 137: 1 Eng. feel. R. 480; 2 E.g. Eccl. R. 13.