- noun a substance such as graphite or heavy water which is used to slow down the speed of the neutrons in a nuclear reactor
- noun a person responsible for reading messages sent to a mailing list or newsgroup and editing any messages that do not conform to the rules of the list, e.g. by deleting commercial messages
- A substance, such as graphite, heavy water, or beryllium, which is utilized in nuclear reactors to slow down neutrons. Also called neutron moderator.