The Collaborative International Dictionary of English v.0.48:

Mediation \Me`di*a"tion\, n. [OE. mediacioun, F. m['e]diation.
   See Mediate, a.]
   1. The act of mediating; action or relation of anything
      interposed; action as a necessary condition, means, or
      instrument; interposition; intervention.
      [1913 Webster]

            The soul [acts] by the mediation of these passions.
                                                  --South.
      [1913 Webster]

   2. Hence, specifically, agency between parties at variance,
      with a view to reconcile them; entreaty for another;
      intercession. --Bacon.
      [1913 Webster]

WordNet (r) 3.0 (2006):

mediation
    n 1: a negotiation to resolve differences that is conducted by
         some impartial party
    2: the act of intervening for the purpose of bringing about a
       settlement [syn: mediation, intermediation]

Moby Thesaurus II by Grady Ward, 1.0:

29 Moby Thesaurus words for "mediation":
   United Nations troops, agency, appeasement, calming, conciliation,
   detente, device, dulcification, easing of relations, expedient,
   going between, instrumentality, intermediation, machinery,
   mechanism, mollification, pacification, peace-keeping force,
   peacemaking, peacemongering, placability, placation, propitiation,
   recourse, relaxation of tension, resort, service, soothing,
   tranquilization



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

MEDIATION. The act of some mutual friend of two contending parties, who
brings them to agree, compromise or settle their disputes. Vattel, Droit des
Gens, liv. 2, eh. 18, Sec. 328.