The Collaborative International Dictionary of English v.0.48:

Mediate \Me"di*ate\, v. i. [imp. & p. p. Mediated; p. pr. &
   vb. n. Mediating.] [LL. mediatus, p. p. of mediare to
   mediate. See Mediate, a.]
   [1913 Webster]
   1. To be in the middle, or between two; to intervene. [R.]
      [1913 Webster]

   2. To interpose between parties, as the equal friend of each,
      esp. for the purpose of effecting a reconciliation or
      agreement; as, to mediate between nations.
      [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

37 Moby Thesaurus words for "mediating":
   agential, agentival, agentive, arbitrational, arbitrative,
   assisting, coming between, conducive, employable, facilitating,
   favoring, forwarding, going between, handy, helpful, instrumental,
   intercessional, intercessory, interlocutory, intermedial,
   intermediary, intermediate, intermediatory, intervening,
   interventional, mediative, mediatorial, mediatory, mesne, middle,
   ministerial, ministering, pacificatory, promoting, serviceable,
   subservient, useful