The ratio of the intensity of radiation scattered at an angle of observation θ to the intensity of radiation scattered at an angle zero, i.e. P θ ≡ R θ R 0. An alternative recommended symbol is P θ. Synonymous with particle scattering factor.
Source:
Purple Book, 1st ed., p. 66 (http://old.iupac.org/publications/books/author/metanomski.html)