fugacity, \(f\), \(\tilde{p}\)

https://doi.org/10.1351/goldbook.F02543
Of a substance B, fB or p(tilde)B, in a gaseous mixture is defined by fb = λB lim(p->0).pB/λB, where pB is the partial pressure of B and λB its absolute activity.
Source:
Green Book, 2nd ed., p. 50 (https://dev.goldbook.iupac.org/files/pdf/green_book_2ed.pdf)