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

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]

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.

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