https://doi.org/10.1351/goldbook.A00115
Defined by the equation \[a = e^{\frac{\mu - \mu^{o}}{R\,T}}\] where \(R\) is the gas constant, \(T\) the thermodynamic temperature, \(\mu\) the chemical potential and \(\mu^{o}\) the standard chemical potential the definition of which depends on the choice of the standard state.
See also: absolute activity