turbidity, \(\tau\)

in light scattering
https://doi.org/10.1351/goldbook.T06533
The apparent absorbance of the incident radiation due to scattering. For small particles, direct proportionality exists between turbidity and the Rayleigh ratio.
Source:
Purple Book, 1st ed., p. 66 (http://old.iupac.org/publications/books/author/metanomski.html)