(i) A dimensionless measure of the deviation of the chemical potential of a substance from its value at a standard state. It is defined by the equation: µ = µ°+ RT ln a, where µ is the chemical potential at activity = a, µ°is the chemical potential in the standard state (where a = 1. 0), R is the molar gas constant, and T is the absolute temperature. In solution a = molal concentration at infinite dilution (molal concentration = molar concentration at low concentrations), and in gases a = partial pressure in atmospheres. (ii) Informally, in solution, it may be taken as the effective concentration of a substance.