moderator

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun One that moderates, as.
• noun One that arbitrates or mediates.
• noun One who presides over a meeting, forum, or debate.
• noun The officer who presides over a synod or general assembly of the Presbyterian Church.
• noun Physics A substance, such as water or graphite, used in a nuclear reactor to decrease the speed of fast neutrons and increase the likelihood of fission.

from The Century Dictionary.

• noun In astrology, any one of the four principal points in a nativity upon which the native's fortunes are supposed to depend: the sun, moon, ascendant, or mid-heaven.
• noun In mech., a device for moderating or reducing the speed of an internal-combustion engine by the operation of a throttle-valve. Its action is the reverse of that of the accelerator.
• noun One who or that which moderates, restrains, or represses.
• noun In microscopy, a device used to diminish the intensity or vary the character of the light which illuminates the object: it consists commonly of a screen of opal glass, ground glass, or glass of a pale-blue or neutral tint.
• noun An umpire; a judge.
• noun The person who presides at a meeting or disputation: now used chiefly in churches of the Presbyterian and Congregational order (as, the moderator of a presbytery or of the General Assembly), and in town-meetings in the United States.
• noun In the universities of Cambridge and Oxford, one of the public officers appointed to superintend the examinations for honors and degrees: so called because they formerly had to moderate or preside in the exercises of undergraduates for the degree of bachelor of arts.
• noun A moderator-lamp.

from the GNU version of the Collaborative International Dictionary of English.

• noun One who, or that which, moderates, restrains, or pacifies.
• noun The officer who presides over an assembly or discussion to preserve order, propose questions, regulate the proceedings, and declare the votes.
• noun 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.
• noun A mechanical arrangement for regulating motion in a machine, or producing equality of effect.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun someone who moderates
• noun the person who presides over a synod of a Presbyterian Church
• noun physics a substance (often water or graphite) used to decrease the speed of fast neutrons in a nuclear reactor and hence increase likelihood of fission
• noun a device used to deaden some of the noise from a firearm, although not to the same extent as a suppressor or silencer.
• noun UK An examiner at Oxford and Cambridge universities.
• noun Ireland At the University of Dublin, either the first (senior) or second (junior) in rank in an examination for the degree of Bachelor of Arts.
• noun A mechanical arrangement for regulating motion in a machine, or producing equality of effect.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun someone who mediates disputes and attempts to avoid violence
• noun any substance used to slow down neutrons in nuclear reactors
• noun in the Presbyterian church, the officer who presides over a synod or general assembly
• noun someone who presides over a forum or debate