Source: cirosantilli/thermal-neutron

= Thermal neutron

These are <neutrons> that have reached the thermal equilibrium according to the <Maxwell-Boltzmann distribution> after having bounced around many times without undergoing <neutron capture>.

Good <fissile material> is material that is able to absorb <thermal neutrons> and continue the reaction, because that's the type of neutron you end up getting the most of.