Defined by the equation \[a = e^{\frac{\mu - \mu^{o}}{R\,T}}\] where R is the gas constant, T the thermodynamic temperature, µ the chemical potential and µ0 the standard chemical potential the definition of which depends on the choice of the standard state.
See also: absolute activity
Sources:
Green Book, 2nd ed., p. 49 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
PAC, 1996, 68, 957. 'Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)' on page 989 (https://doi.org/10.1351/pac199668040957)