The activity coefficient fB of a substance B in a liquid or solid mixture containing mole fractions x B, x C, ... of the substances B, C, ... is a dimensionless quantity defined in terms of the chemical potential µ B of B in the mixture by: \[R\ T\ \ln (x_{\text{B}}\ f_{\text{B}})=\mu _{\text{B}}\left(\text{cd},\,T,\,P,\,x\right)- \mu _{\text{B}}^{*}\left(\text{cd},\,T,\,p\right)\] where x denotes the set of mole fractions x B, x C, ... . The activity coefficient γ B of a solute B in a solution (especially a dilute liquid solution) containing molalities m B, m C, ... of solutes B, C, ... in a solvent A is a dimensionless quantity defined in terms of the chemical potential µ B by: \[R\ T\ \ln \left(\frac{m_{\text{B}}\ \gamma _{\text{B}}}{m^{\ominus}}\right)=\mu _{\text{B}}- \left\{\mu _{\text{B}}- R\ T\ \ln \left(\frac{m_{\text{B}}}{m^{\ominus }}\right)\right\}^{\infty }\]