Mean-field theory of liquid crystals: rod-like molecules align collectively below the isotropic-nematic transition temperature.
The nematic order parameter S = ½⟨3cos²θ - 1⟩ measures orientational order (S=0: isotropic, S=1: perfect alignment). Maier-Saupe: first-order transition at T* with S jumping discontinuously to ≈0.43.