a person or thing that moderates.
a person who presides over a panel discussion on radio or television.
a presiding officer, as at a public forum, a legislative body, or an ecclesiastical body in the Presbyterian Church.
Physics. a substance, as graphite or heavy water, used to slow neutrons to speeds at which they are more efficient in causing fission.

1350–1400; Middle English < Latin moderātor, equivalent to moderā() to control (see moderate) + -tor -tor

moderatorial [mod-er-uh-tawr-ee-uhl, -tohr-] , adjective
moderatorship, noun Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
moderator (ˈmɒdəˌreɪtə)
1.  a person or thing that moderates
2.  Presbyterian Church a minister appointed to preside over a Church court, synod, or general assembly
3.  a presiding officer at a public or legislative assembly
4.  a material, such as heavy water or graphite, used for slowing down neutrons in the cores of nuclear reactors so that they have more chance of inducing nuclear fission
5.  an examiner at Oxford or Cambridge Universities in first public examinations
6.  (in Britain and New Zealand) one who is responsible for consistency of standards in the grading of some educational assessments
7.  a person who monitors the conversations in an on-line chatroom for bad language, inappropriate content, etc

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

late 14c., "ruler, governor," from L. moderator "manager, ruler, director," lit. "he who moderates," from moderatus (see moderate (adj.)). Meaning "one who acts as an umpire" is from c.1560.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Science Dictionary
moderator   (mŏd'ə-rā'tər)  Pronunciation Key 
A substance, such as graphite, water, or heavy water, placed in a nuclear reactor to slow neutrons down to speeds at which they are more likely to be captured by fissionable components of a fuel (such as uranium-235) and less likely to be absorbed by nonfissionable components of a fuel (such as uranium-238). Also called neutron moderator. See also slow neutron.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Computing Dictionary

moderator definition

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.

The Free On-line Dictionary of Computing, © Denis Howe 2010
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Example sentences
My posting on this forum was deleted by the moderator.
Graphite is a great moderator, slowing down the neutrons and keeping their
  reaction in the proper temperature range.
The entire staff is trying to write a mission statement with help from a
Also, the moderator's questions to both candidates are going to be the same, it
  shouldn't matter.
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