Definition of: moderator

(mod′ə·rā′tər) noun
1. One who restrains or regulates.
2. The presiding officer of a meeting; also, the presiding officer in Presbyterian and Congregational courts.
3. Physics A substance, as graphite or beryllium, used to control the rate of a nuclear chain reaction in an atomic–energy reactor.
—mod′er·a′tor·ship noun