The Collaborative International Dictionary of English v.0.48:

Supplication \Sup`pli*ca"tion\, n. [F. supplication, L.
   supplicatio.]
   1. The act of supplicating; humble and earnest prayer, as in
      worship.
      [1913 Webster]

   2. A humble petition; an earnest request; an entreaty.
      [1913 Webster]

   3. (Rom. Antiq.) A religious solemnity observed in
      consequence of some military success, and also, in times
      of distress and danger, to avert the anger of the gods.
      [1913 Webster]

   Syn: Entreaty; petition; solicitation; craving.
        [1913 Webster]

WordNet (r) 3.0 (2006):

supplication
    n 1: a prayer asking God's help as part of a religious service
         [syn: invocation, supplication]
    2: a humble request for help from someone in authority [syn:
       supplication, plea]
    3: the act of communicating with a deity (especially as a
       petition or in adoration or contrition or thanksgiving); "the
       priest sank to his knees in prayer" [syn: prayer,
       supplication]

Moby Thesaurus II by Grady Ward, 1.0:

55 Moby Thesaurus words for "supplication":
   Angelus, Ave, Ave Maria, Hail Mary, Kyrie Eleison, Paternoster,
   adjuration, aid prayer, appeal, application, beadroll, beads,
   begging, beseeching, beseechment, bid, bidding prayer, breviary,
   call, chaplet, clamor, collect, communion, contemplation, cry,
   devotions, entreating, entreaty, grace, impetration, imploration,
   imploring, imprecation, intercession, invocation, invocatory plea,
   litany, meditation, obsecration, obtestation, orison, petition,
   plea, pleading, prayer, prayer wheel, rogation, rosary,
   silent prayer, solicitation, soliciting, suit, supplicating,
   thanks, thanksgiving