NOUN (3)

1. a member of a military force that is assigned (often with international sanction) to preserve peace in a trouble area;

2. someone who keeps peace;
- Example: "she's the peacekeeper in that family"

3. the pistol of a law officer in the old West;

The Collaborative International Dictionary of English v.0.48:

peacekeeper \peacekeeper\ n.
   1. a member of a military force that is assigned (often with
      international sanction) to preserve peace in a trouble
      area.
      [WordNet 1.5]

   2. someone who keeps peace; as, she's the peacekeeper in that
      family.
      [WordNet 1.5]

   3. the pistol of a law officer in the old West. [U. S.
      History]
      [WordNet 1.5]

WordNet (r) 3.0 (2006):

peacekeeper
    n 1: a member of a military force that is assigned (often with
         international sanction) to preserve peace in a trouble area
    2: someone who keeps peace; "she's the peacekeeper in that
       family"
    3: the pistol of a law officer in the old West