decay constant
Proportionality constant relating the activity of a radioactive substance, A, to the number of decaying particles B, Math - text, Math - ei. Probability of a nuclear decay within a time interval divided by that time interval. Synonymous with disintegration constant.
Green Book, p. 22
See also:
PAC, 1994, 66, 2513 (Nomenclature for radioanalytical chemistry (IUPAC Recommendations 1994)) on page 2518
PAC, 1996, 68, 957 (Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)) on page 968