The Collaborative International Dictionary of English v.0.48:

Peace \Peace\, n. [OE. pees, pais, OF. pais, paiz, pes, F. paix,
   L. pax, pacis, akin to pacere, paciscere, pacisci, to make an
   agreement, and prob. also pangere to fasten. Cf. Appease,
   Fair, a., Fay, v., Fang, Pacify, Pact, Pay to
   requite.]
   A state of quiet or tranquillity; freedom from disturbance or
   agitation; calm; repose; specifically:
   (a) Exemption from, or cessation of, war with public enemies.
   (b) Public quiet, order, and contentment in obedience to law.
   (c) Exemption from, or subjection of, agitating passions;
       tranquillity of mind or conscience.
   (d) Reconciliation; agreement after variance; harmony;
       concord. "The eternal love and pees." --Chaucer.
       [1913 Webster]

   Note: Peace is sometimes used as an exclamation in commanding
         silence, quiet, or order. "Peace! foolish woman."
         --Shak.
         [1913 Webster]

   At peace, in a state of peace.

   Breach of the peace. See under Breach.

   Justice of the peace. See under Justice.

   Peace of God. (Law)
   (a) A term used in wills, indictments, etc., as denoting a
       state of peace and good conduct.
   (b) (Theol.) The peace of heart which is the gift of God.

   Peace offering.
   (a) (Jewish Antiq.) A voluntary offering to God in token of
       devout homage and of a sense of friendly communion with
       Him.
   (b) A gift or service offered as satisfaction to an offended
       person.

   Peace officer, a civil officer whose duty it is to preserve
      the public peace, to prevent riots, etc., as a polliceman,
      sheriff or constable.

   To hold one's peace, to be silent; to refrain from
      speaking.

   To make one's peace with, to reconcile one with, to plead
      one's cause with, or to become reconciled with, another.
      "I will make your peace with him." --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

at peace
    adj 1: dead; "he is deceased"; "our dear departed friend" [syn:
           asleep(p), at peace(p), at rest(p), deceased,
           departed, gone]