decay constant

https://doi.org/10.1351/goldbook.D01538
Proportionality constant relating the activity of a radioactive substance, A, to the number of decaying particles B, N B,A = λNB. probability of a nuclear decay within a time interval divided by that time interval. Synonymous with disintegration constant.
Source:
Green Book, 2nd ed., p. 22 (https://dev.goldbook.iupac.org/files/pdf/green_book_2ed.pdf)