In a given experimental configuration, the energy value determined by the condition that the detector response would be unchanged if the cadmium cover surrounding the detector was replaced by a fictitious cover opaque to neutrons with energy below this value and transparent to neutrons with energy above this value.
Sources:
Orange Book, 2nd ed., p. 221 (https://media.iupac.org/publications/analytical_compendium/)
PAC, 1982, 54, 1533. 'Glossary of terms used in nuclear analytical chemistry (Provisional)' on page 1540 (https://doi.org/10.1351/pac198254081533)