Fugacity can be thought of as the activity of a gas. The fugacity of the `i^(th)` component in a mixture at equilibrium is the same whether in vapor or liquid phase. For an Ideal Gas fugacity is equal to Pressure. This equation defines the fugacity of `i^(th)` component in a liquid mixture using the component characteristics of: mole fraction, activity coefficient, and fugacity of the pure liquid component.
The activity coefficient `gamma_i`, is a correction for liquid phase non-ideality for the `i^(th)` component of the mixture. Many models have been proposed for `gamma_i` such as the Van Laar model.