Source : Webster's Revised Unabridged Dictionary (1913)

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

            Sound is shut out by the intervention of that lax
            membrane.                             --Holder.

   2. Any interference that may affect the interests of others;
      especially, of one or more states with the affairs of
      another; mediation.

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

   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.

Source : WordNet®

intervention
     n 1: the act of intervening (as to mediate a dispute) [syn: {intercession}]
     2: a policy of intervening in the affairs of other countries
        [syn: {interference}] [ant: {nonintervention}, {nonintervention}]
     3: (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"