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]