The Collaborative International Dictionary of English v.0.48:

Prayer \Pray"er\, n.
   One who prays; a supplicant.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Prayer \Prayer\ (?; 277), n. [OE. preiere, OF. preiere, F.
   pri[`e]re, fr. L. precarius obtained by prayer, fr. precari
   to pray. See Pray, v. i.]
   1. The act of praying, or of asking a favor; earnest request
      or entreaty; hence, a petition or memorial addressed to a
      court or a legislative body. "Their meek preyere."
      --Chaucer
      [1913 Webster]

   2. The act of addressing supplication to a divinity,
      especially to the true God; the offering of adoration,
      confession, supplication, and thanksgiving to the Supreme
      Being; as, public prayer; secret prayer.
      [1913 Webster]

            As he is famed for mildness, peace, and prayer.
                                                  --Shak.
      [1913 Webster]

   3. The form of words used in praying; a formula of
      supplication; an expressed petition; especially, a
      supplication addressed to God; as, a written or
      extemporaneous prayer; to repeat one's prayers.
      [1913 Webster]

            He made those excellent prayers which were published
            immediately after his death.          --Bp. Fell.
      [1913 Webster]

   Prayer book, a book containing devotional prayers.

   Prayer meeting, a meeting or gathering for prayer to God.
      [1913 Webster]

   Syn: Petition; orison; supplication; entreaty; suit.
        [1913 Webster]

WordNet (r) 3.0 (2006):

prayer
    n 1: 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]
    2: reverent petition to a deity [syn: prayer, petition,
       orison]
    3: earnest or urgent request; "an entreaty to stop the
       fighting"; "an appeal for help"; "an appeal to the public to
       keep calm" [syn: entreaty, prayer, appeal]
    4: a fixed text used in praying
    5: someone who prays to God [syn: prayer, supplicant]