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://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)' on page 569 (https://doi.org/10.1351/pac198456050567)
PAC, 1996, 68, 957. 'Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)' on page 959 (https://doi.org/10.1351/pac199668040957)