"Debye-Waller Factor" (DWF), named after Peter Debye and Ivar Waller, is used in condensed matter physics to describe the attenuation of x-ray scattering or coherent neutron scattering caused by thermal motion. Also, a factor by which the intensity of a diffraction line is reduced because of the lattice vibrations.
$u = R - R_0$, where $R$ is the particle position vector and $R_0$ is the equilibrium position vector of a particle.
Debye-Waller Factor