A thermal neutron belongs to a population approximately in thermal equilibrium with its surroundings. Such a population has a distribution of kinetic energies, not one fixed energy. The scale uses the Boltzmann constant and thermodynamic temperature; it is about at . The mean translational kinetic energy for a classical three-dimensional equilibrium population is . Not every slow neutron has reached this equilibrium.
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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.