What is a moderator? Name the material used as a moderator in a nuclear reactor.

(N/A) moderator is a substance used in a nuclear reactor to slow down the fast-moving neutrons produced during nuclear fission.
Fast neutrons are less likely to cause further fission in $U^{235}$ nuclei compared to thermal (slow) neutrons.
By colliding with the nuclei of the moderator material,the kinetic energy of the neutrons is reduced,making them 'thermal' neutrons.
Common materials used as moderators include heavy water $(D_2O)$,graphite,and ordinary water $(H_2O)$.