Source : Webster's Revised Unabridged Dictionary (1913)

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.

            Angling was . . . a moderator of passions. --Walton.

   2. The officer who presides over an assembly to preserve
      order, propose questions, regulate the proceedings, and
      declare the votes.

   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.

   4. A mechamical arrangement for regulating motion in a
      machine, or producing equality of effect.

Source : WordNet®

moderator
     n 1: any substance used to slow down neutrons in nuclear reactors
     2: in the Presdyterian 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

Source : Free On-Line Dictionary of Computing

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.