a neutron produced by nuclear fission that has lost little energy by collision; a neutron with a kinetic energy in excess of 0.1 MeV
a neutron with a kinetic energy in excess of 1.5 MeV, the fission threshold of uranium-238
fast neutron in Science
fast neutron A neutron that is not in thermal equilibrium with the surrounding medium, especially one produced by fission. Compare slow neutron. See also fast-breeder reactor.