activity (relative activity), a
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.
PAC, 1996, 68, 957 (Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)) on page 989