Variation of a quantity (generally the activity of a quantity of a radionuclide) according to the law A = A0.e(-λt) where A and A0 are the values of the quantity being considered at time t and zero respectively, and λ is an appropriate constant.
Source:
PAC, 1982, 54, 1533. 'Glossary of terms used in nuclear analytical chemistry (Provisional)' on page 1541 (https://doi.org/10.1351/pac198254081533)