A measure of the amount of energy a neutron loses upon colliding with various nuclei. A moderator should have a high &xi, to slow neutrons down quickly. This reduces the neutron's energy (thermalizes) and makes them more likely to cause fissions than fast neutrons, which are those that have an energy level greater than one MeV. It is also good because it reduces neutron leakage (the amount of neutrons that escape from the reactor core), which means more fissions, and we're all about that.