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

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.
Green Book, 2nd ed., p. 50 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
See also:
PAC, 1984, 56, 567. 'Physicochemical quantities and units in clinical chemistry with special emphasis on activities and activity coefficients (Recommendations 1983)' (https://doi.org/10.1351/pac198456050567)
PAC, 1994, 66, 533. 'Standard quantities in chemical thermodynamics. Fugacities, activities and equilibrium constants for pure and mixed phases (IUPAC Recommendations 1994)' (https://doi.org/10.1351/pac199466030533)