exponential decay

Variation of a quantity (generally the activity of a quantity of a radionuclide) according to the law $A = A 0 e − λ t$ where $A$ and $A 0$ 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