NOUN (4)

1. any substance used to slow down neutrons in nuclear reactors;

2. in the Presbyterian church, the officer who presides over a synod or general assembly;

3. someone who presides over a forum or debate;

4. someone who mediates disputes and attempts to avoid violence;

The Collaborative International Dictionary of English v.0.48:

Moderator \Mod"er*a`tor\, n. [L.: cf. F. mod['e]rateur.]
   1. One who, or that which, moderates, restrains, or pacifies.
      --Sir W. Raleigh.
      [1913 Webster]

            Angling was . . . a moderator of passions. --Walton.
      [1913 Webster]

   2. The officer who presides over an assembly or discussion to
      preserve order, propose questions, regulate the
      proceedings, and declare the votes.
      [1913 Webster]

   3. In the University of Oxford, an examiner for moderations;
      at Cambridge, the superintendant of examinations for
      degrees; at Dublin, either the first (senior) or second
      (junior) in rank in an examination for the degree of
      Bachelor of Arts.
      [1913 Webster]

   4. A mechanical arrangement for regulating motion in a
      machine, or producing equality of effect.
      [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

57 Moby Thesaurus words for "moderator":
   JP, Justice, MC, alleviative, alleviator, anchorman, anodyne,
   arbiter, arbitrator, assuager, balm, bencher, calmative, chair,
   chairman, conciliator, coordinator, critic, cushion, dolorifuge,
   emcee, his honor, his lordship, his worship, impartial arbitrator,
   indicator, judge, justice, lenitive, magistrate,
   master of ceremonies, mediator, mitigator, modulator, mollifier,
   negotiator, pacificator, pacifier, palliative, peacemaker,
   president, presiding officer, referee, restraining hand, salve,
   sedative, shock absorber, soother, soothing syrup, stabilizer,
   temperer, third party, toastmaster, tranquilizer, umpire,
   unbiased observer, wiser head