The Collaborative International Dictionary of English v.0.48:

Intercessor \In`ter*ces"sor\, n. [L., a surety: cf. F.
   intercesseur.]
   [1913 Webster]
   1. One who goes between, or intercedes; a mediator.
      (a) One who interposes between parties at variance, with a
          view to reconcile them.
      (b) One who pleads in behalf of another. --Milton.
          [1913 Webster]

   2. (Eccl.) A bishop, who, during a vacancy of the see,
      administers the bishopric till a successor is installed.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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