moderator

In a nuclear reactor, a material such as graphite or heavy water used to reduce the speed of high-energy neutrons. Neutrons produced by nuclear fission are fast-moving and must be slowed to initiate further fission so that nuclear energy continues to be released at a controlled rate.

Slow neutrons are much more likely to cause fission in a uranium-235 nucleus than to be captured in a U-238 (nonfissile uranium) nucleus. By using a moderator, a reactor can thus be made to work with fuel containing only a small proportion of U-235.

moderator

In computing, person or group of people that screens submissions to certain newsgroups, forums, and mailing lists before passing them on for wider circulation. The aim of moderation is not to censor, but to ensure that the quality of debate is maintained by filtering out spamming, irrelevant (‘off-topic’), or gratuitously offensive postings.