- a neutron with low kinetic energy, especially one slowed by the moderator in a nuclear reactor.
- physics a neutron having a kinetic energy of less than 100 electronvolts
- See slow neutron.
- A neutron that is in thermal equilibrium with the surrounding medium, especially one produced by fission in a nuclear reactor, and slowed by a moderator. The uranium 235 used in thermal nuclear reactors undergoes fission more readily when struck by slow neutrons than when struck by the fast neutrons released by fission. Also called thermal neutron Compare fast neutron.