specific activity, a in radiochemistry

https://dx.doi.org/10.1351/goldbook.S05790
For a specified isotope, or mixture of isotopes, the activity of a material divided by the mass of the material.
Source:
PAC, 1994, 66, 2513 (Nomenclature for radioanalytical chemistry (IUPAC Recommendations 1994)) on page 2515
PAC, 1996, 68, 957 (Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)) on page 993