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, $$N_{\text{B}}$$,$$A=\lambda \ N_{\text{B}}$$. 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://www.cintegrado.com.br/site/documentos/green_book_2ed.pdf)