Neutrons of kinetic energy between the energies of slow and fast neutrons. In reactor physics, the range might be 1 eV to 0.1 MeV.
Source:
PAC, 1982, 54, 1533. 'Glossary of terms used in nuclear analytical chemistry (Provisional)' on page 1547 (https://doi.org/10.1351/pac198254081533)