moderator

mod·er·a·tor   (mŏd'ə-rā'tər)    n.   One that moderates, as: One that arbitrates or mediates. One who presides over a meeting, forum, or debate. The officer who presides over a synod or general assembly of the Presbyterian Church. Physics A substance, such as water or graphite, that is used in a nuclear reactor to decrease the speed of fast neutrons and increase the likelihood of fission.

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.

moderator 

Main Entry: mod·er·a·tor
Pronunciation: 'mäd-&-"rAt-&r
Function: noun
: a substance (as graphite, deuterium in heavy water,or beryllium) used for slowing down neutrons in a nuclear reactor

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.

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.