Neutrons whose energy corresponds to the resonance energy of a specified nuclide or element. If the nuclide is not specified, the term refers to resonance neutrons of $\ce{^{239}U}$.
Source: PAC, 1994, 66, 2513. (Nomenclature for radioanalytical chemistry (IUPAC Recommendations 1994)) on page 2522 [Terms] [Paper]