https://doi.org/10.1351/goldbook.R05405

The square root of the mean-square end-to-end distance of a linear polymer chain averaged over all conformations of the chain. For a freely jointed chain consisting of N segments each of length L, it is given by: \[\text{<}r^{2}\text{>}^{1/2} = \sqrt{N}\ L\] The subscript zero is used to indicate unperturbed dimensions, as in < r2 >0(1/2). If this term is used repeatedly, and if it is not confusing, the abbreviated name 'end-to-end distance' may be used.*Source: *

Purple Book, 1^{st} ed., p. 49 (http://old.iupac.org/publications/books/author/metanomski.html)

Purple Book, 1