moderator

Concise Oxford English Dictionary © 2008 Oxford University Press:
moderator/ˈmɒdəreɪtə(r)/

▶noun

• 1 an arbitrator or mediator.

  ■ a presiding officer, especially a chairman of a debate.

  ■ a Presbyterian minister presiding over an ecclesiastical body.

• 2 Brit. a person who moderates examination papers.

• 3 Physics a substance used in a nuclear reactor to retard neutrons.

• 4 a person who moderates an Internet bulletin board or chat room.

– derivatives
moderatorship noun.