moderator

from WordNet (r) 3.0 (2006)

moderator
    n 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

from 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]

from The Free On-line Dictionary of Computing (8 July 2008)

moderator

   A person, or small group of people, who manages a moderated
   {mailing list} or {Usenet} {newsgroup}.  Moderators are
   responsible for determining which email submissions are passed
   on to the list or newsgroup.

from Bouvier's Law Dictionary, Revised 6th Ed (1856)

MODERATOR. A person appointed to preside at a popular meeting; sometimes he 
is called a chairman.

from 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