rotational diffusion coefficient 

https://doi.org/10.1351/goldbook.R05411
Defined by the equation: Dθ=tθ(f(θ,Φ))θ sinθ where f(θ,Φ)sinθ dθ dΦ is the fraction of particles whose axes make an angle between θ and θ+dθ with the direction θ=0, and have an azimuth between Φ and Φ+dΦ; tθ dΦ is the fraction of particles having an azimuth between Φ and Φ+dΦ whose axis passes from values <θ to values >θ in unit time. The axis whose rotational diffusion is considered has to be clearly indicated.
Source:
PAC, 1972, 31, 577. (Manual of Symbols and Terminology for Physicochemical Quantities and Units, Appendix II: Definitions, Terminology and Symbols in Colloid and Surface Chemistry) on page 617 [Terms] [Paper]