Defined by the equation: \[D_{\theta }=\frac{t_{\theta }}{\frac{\partial (f(\theta ,\mathit{\Phi }))}{\partial \theta }\ \sin\,\theta }\] where f(θ,Φ).sin θ.dθ.dΦ is the fraction of particles whose axes make an angle between θ and θ + d θ with the direction θ = 0, and have an azimuth between Φ and Φ + d Φ; t θ d Φ is the fraction of particles having an azimuth between Φ and Φ + d Φ whose axis passes from values <θ to values >θ in unit time. The axis whose rotational diffusion is considered has to be clearly indicated.