For a specified radiation, the minimum thickness of a flat preparation of a radioactive material where the intensity of the specified radiation at the surface does not increase when the thickness is increased by adding more of similar radioactive material.
Source:
PAC, 1982, 54, 1533. 'Glossary of terms used in nuclear analytical chemistry (Provisional)' on page 1544 (https://doi.org/10.1351/pac198254081533)