A continuous distribution with the differential mass-distribution function of the form: \[f_{\text{w}}(x)\ \text{d}x = \frac{1}{a\ \sqrt{\pi \ x}}\ \exp (- \frac{1}{a^{2}}\ \ln ^{2}\frac{x}{b})\ \text{d}x\] where x is a parameter characterizing the chain length, such as relative molecular mass or degree of polymerization and a and b are positive adjustable parameters.
Source:
Purple Book, 1st ed., p. 57 (http://old.iupac.org/publications/books/author/metanomski.html)