## fugacity $$f$$, $$\tilde{p}$$

https://doi.org/10.1351/goldbook.F02543
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.
Source:
Green Book,2nd ed., p. 50 (https://www.cintegrado.com.br/site/documentos/green_book_2ed.pdf)