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.
      [1913 Webster]

   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.
      [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

intervening
    adj 1: occurring or falling between events or points in time;
           "so much had happened during the intervening years"

Moby Thesaurus II by Grady Ward, 1.0:

26 Moby Thesaurus words for "intervening":
   arbitrational, arbitrative, coming between, going between,
   intercessional, intercessory, intercurrent, interjacent,
   interlocutory, intermedial, intermediary, intermediate,
   intermediatory, intervenient, interventional, mean, medial, median,
   mediating, mediative, mediatorial, mediatory, medium, mesne,
   middle, pacificatory