The Collaborative International Dictionary of English v.0.48:

Mediator \Me"di*a`tor\, n. [L. mediator: cf. E. m['e]diateur.]
   One who mediates; especially, one who interposes between
   parties at variance for the purpose of reconciling them;
   hence, an intercessor.
   [1913 Webster]

         For there is one God, and one mediator between God and
         men, the man Christ Jesus.               --1 Tim. ii.
                                                  5.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mediator
    n 1: a negotiator who acts as a link between parties [syn:
         mediator, go-between, intermediator, intermediary,
         intercessor]