root-mean-square end-to-end distance, <r2>1/2

in polymers
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: <r2>1/2=N L The subscript zero is used to indicate unperturbed dimensions, as in <r2>01/2. 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, 1st ed., p. 49 [Terms] [Book]