What is Activity coefficient?

In a real mixture the chemical potential is written $\mu_i = \mu_i^\circ + RT\ln a_i$ with $a_i = \gamma_i x_i$ (Raoult reference) or $a_i = \gamma_i c_i/c^\circ$ (Henry reference). The factor $\gamma_i$ is the activity coefficient: it measures the departure from the ideal law, and tends to 1 in the limit where the reference law holds.