## absolute activity, $$\lambda$$

https://doi.org/10.1351/goldbook.A00019
The exponential of the ratio of the chemical potential, µ, to RT where R is the gas constant and T the thermodynamic temperature, λ = e^(µ/RT).
Source:
Green Book, 2nd ed., p. 40 (https://dev.goldbook.iupac.org/files/pdf/green_book_2ed.pdf)