The ratio of the appropriate value of the biological effectiveness of the radiation in question to that of X-radiation with an average specific ionization of 100 ion pairs per micron of water, for the particular biological effect under consideration and for the condition under which the radiation is received.
Source:
PAC, 1982, 54, 1533. 'Glossary of terms used in nuclear analytical chemistry (Provisional)' on page 1551 (https://doi.org/10.1351/pac198254081533)