The number of nuclear decays occurring in a given quantity of material in a small time interval, divided by that time interval. Often, this term is referred to as absolute activity. Symbol A = -dN/dt. Synonymous with disintegration rate or decay rate.
Sources:
Green Book, 2nd ed., p. 22 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
PAC, 1994, 66, 2513. 'Nomenclature for radioanalytical chemistry (IUPAC Recommendations 1994)' on page 2515 (https://doi.org/10.1351/pac199466122513)
PAC, 1996, 68, 957. 'Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)' on page 960 (https://doi.org/10.1351/pac199668040957)