Energy imparted to matter by ionizing radiation in a suitable small element of volume divided by the mass of that element of volume.
Sources:
Orange Book, 2nd ed., p. 220 (https://media.iupac.org/publications/analytical_compendium/)
PAC, 1993, 65, 2003. 'Glossary for chemists of terms used in toxicology (IUPAC Recommendations 1993)' on page 2007 (https://doi.org/10.1351/pac199365092003)
PAC, 1996, 68, 957. 'Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)' on page 959 (https://doi.org/10.1351/pac199668040957)