Measure for integrated intensity of electronic transitions and related to the Einstein transition probability coefficient Ai j by: \[f_{ij} = 1.4992\times 10^{-14}\ \frac{A_{ij}}{s^{-1}}\ (\frac{\lambda }{\text{nm}})^{2}\] where λ is the transition wavelength. There are, however, differing uses of f.
Source:
Green Book, 2nd ed., p. 33 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
See also:
PAC, 1996, 68, 2223. 'Glossary of terms used in photochemistry (IUPAC Recommendations 1996)' on page 2257 (https://doi.org/10.1351/pac199668122223)