noun

• Someone who moderates.
• An arbitrator or mediator.
• The chair or president of a meeting, etc.
• (Internet) A person who enforces the rules of a discussion forum by deleting posts, banning users, etc.
• The person who presides over a synod of a Presbyterian Church.
• (nuclear 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.
• A device used to deaden some of the noise from a firearm, although not to the same extent as a suppressor or silencer.
• (UK) An examiner at Oxford and Cambridge universities.
• (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.
• (UK) Someone who supervises and monitors the setting and marking of examinations by different people to ensure consistency of standards.
• A mechanical arrangement for regulating motion in a machine, or producing equality of effect.
• (historical) A kind of lamp in which the flow of the oil to the wick is regulated.