exponential decay
Variation of a quantity (generally the activity of a quantity of a radionuclide) according to the law Math - ei where Math - text and Math - text are the values of the quantity being considered at time Math - text and zero respectively, and Math - text is an appropriate constant.
PAC, 1982, 54, 1533 (Glossary of terms used in nuclear analytical chemistry (Provisional)) on page 1541