The Collaborative International Dictionary of English v.0.48:

Intercession \In`ter*ces"sion\, n. [L. intercessio an
   intervention, a becoming surety: cf. F. intercession. See
   Intercede.]
   The act of interceding; mediation; interposition between
   parties at variance, with a view to reconcilation; prayer,
   petition, or entreaty in favor of, or (less often) against,
   another or others.
   [1913 Webster]

         But the Spirit itself maketh intercession for us with
         groanings which can not be uttered.      --Rom. viii.
                                                  26.
   [1913 Webster]

WordNet (r) 3.0 (2006):

intercession
    n 1: a prayer to God on behalf of another person
    2: the act of intervening (as to mediate a dispute, etc.); "it
       occurs without human intervention" [syn: intervention,
       intercession]