Pronunciation: (mod'u-rā"tur),[key] —n. 1. a person or thing that moderates.
2. a person who presides over a panel discussion on radio or television.
3. a presiding officer, as at a public forum, a legislative body, or an ecclesiastical body in the Presbyterian Church.
4. Physics.a substance, as graphite or heavy water, used to slow neutrons to speeds at which they are more efficient in causing fission.