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]

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "intercession":
   Angelus, Ave, Ave Maria, Hail Mary, Kyrie Eleison, Paternoster,
   aid prayer, appeal, beadroll, beads, beseechment, bidding prayer,
   breviary, chaplet, collect, communion, contemplation, devotions,
   entreaty, grace, impetration, imploration, interagency,
   intermediation, interposition, intervention, invocation,
   involvement, litany, mediation, meditation, obsecration,
   obtestation, orison, petition, prayer, prayer wheel, rogation,
   rosary, silent prayer, stepping in, suit, supplication, thanks,
   thanksgiving