The Collaborative International Dictionary of English v.0.48:

Intervene \In`ter*vene"\, v. i. [imp. & p. p. Intervened; p.
   pr. & vb. n. Intervening.] [L. intervenire, interventum, to
   intervene, to hinder; inter between + venire to come; akin to
   E. come: cf. F. intervenir. See Come.]
   [1913 Webster]
   1. To come between, or to be between, persons or things; --
      followed by between; as, the Mediterranean intervenes
      between Europe and Africa.
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   2. To occur, fall, or come between, points of time, or
      events; as, an instant intervened between the flash and
      the report; nothing intervened ( i. e., between the
      intention and the execution) to prevent the undertaking.
      [1913 Webster]

   3. To interpose; as, to intervene to settle a quarrel.
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   4. In a suit to which one has not been made a party, to put
      forward a defense of one's interest in the subject matter.
      --Abbott.
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The Collaborative International Dictionary of English v.0.48:

Intervene \In`ter*vene"\, v. t.
   To come between. [R.]
   [1913 Webster]

         Self-sown woodlands of birch, alder, etc., intervening
         the different estates.                   --De Quincey.
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The Collaborative International Dictionary of English v.0.48:

Intervene \In`ter*vene"\, n.
   A coming between; intervention; meeting. [Obs.] --Sir H.
   Wotton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

intervene
    v 1: get involved, so as to alter or hinder an action, or
         through force or threat of force; "Why did the U.S. not
         intervene earlier in WW II?" [syn: intervene, step in,
         interfere, interpose]
    2: be placed or located between other things or extend between
       spaces and events; "This interludes intervenes between the
       two movements"; "Eight days intervened"
    3: occur between other event or between certain points of time;
       "the war intervened between the birth of her two children"