Moderator

noun

1. Someone who moderates
2. The person who presides over a synod of a Presbyterian Church
3. A substance (often water or graphite) used to decrease the speed of fast neutrons in a nuclear reactor and hence increase likelihood of fission
4. A device used to deaden some of the noise from a firearm, although not to the same extent as a suppressor or silencer.
5. An examiner at Oxford and Cambridge universities.