root-mean-square end-to-end distance,

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:

$< r 2 > 1 / 2 = N L$

The subscript zero is used to indicate unperturbed dimensions, as in $< r 2 > 0 1 / 2$ . 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, p. 49