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

Of a substance B, \(f_{\text{B}}\) or \(\tilde{p}_{\text{B}}\), in a gaseous mixture is defined by \(f_{\text{B}}=\lambda _{\text{B}}\ \lim _{p\rightarrow 0}\frac{p_{\text{B}}}{\lambda _{\text{B}}}\), where \(p_{\text{B}}\) is the partial pressure of B and \(λ_{\text{B}}\) its absolute activity.
Green Book,2nd ed., p. 50 (https://www.cintegrado.com.br/site/documentos/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)' on page (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)' on page (https://doi.org/10.1351/pac199466030533)