a person or thing that moderates.

a person who presides over a panel discussion on radio or television.

a presiding officer, as at a public forum, a legislative body, or an ecclesiastical body in the Presbyterian Church.

Physics . a substance, as graphite or heavy water, used to slow neutrons to speeds at which they are more efficient in causing fission.

noun

alleviator , mediator , pacifier , peacemaker , referee , soother , stabilizer , arbitrator , chairman , chairwoman , judge , umpire