root-mean-square end-to-end distance, Math - ei
in polymers
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 Math - text segments each of length Math - text, it is given by:
Math - e
The subscript zero is used to indicate unperturbed dimensions, as in Math - text. 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