Defined by the equation:
where
is the fraction of particles whose axes make an angle between
and
with the direction
,
and have an azimuth between
and
;
is the fraction of particles having an azimuth between
and
whose axis passes from values
to values
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