A thermal neutron is a free neutron with a kinetic energy level of less than 0.025 eV (approx. 4.0e-21 J). They are named 'thermal' as this level of kinetic energy is similar to the average kinetic energy of a room-temperature gas.
Thermal neutrons have a much larger effective cross-section than fast neutrons, and can therefore be absorbed more easily by any atomic nuclei that they collide with, creating a heavier - and often unstable - isotope of the element as a result.
