activity (relative activity), a

https://dx.doi.org/10.1351/goldbook.A00115
Defined by the equation
a = e μ − μ 0 R T
where R is the gas constant, T the thermodynamic temperature, μ the chemical potential and μ 0 the standard chemical potential the definition of which depends on the choice of the standard state.
Source:
PAC, 1996, 68, 957 (Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)) on page 989