A discrete distribution with the differential mass-distribution function of the form: \[f_{\text{w}}(x)=\frac{x}{a\,+\,1}=\frac{\text{e}^{-a}\ a^{x-1}}{(x\,-\,1)!}\] where x is a parameter characterizing the chain length, such as relative molecular mass or degree of polymerization and a is a positive adjustable parameter.
Source:
Purple Book, 1st ed., p. 56 (http://old.iupac.org/publications/books/author/metanomski.html)