Moderator
A material used in a reactor to slow fast neutrons to energies at which fission is far more probable.
Unit
Dimensionless
In depth
Neutrons lose most energy in collisions with nuclei of similar mass, so light elements make the best moderators, provided they do not absorb neutrons. Slow, thermal neutrons are hundreds of times more likely to cause fission in uranium-235 than fast ones.
Examples in the real world
Ordinary water in most reactors, heavy water in CANDU designs, and graphite in early reactors.