The characteristic length defined by the variance of the Gaussian function fitted to the gradient of the electron density profile established by X-ray specular reflectivity studies of the liquid interface.
Source:
PAC, 1994, 66, 1667. 'Thin films including layers: terminology in relation to their preparation and characterization (IUPAC Recommendations 1994)' on page 1674 (https://doi.org/10.1351/pac199466081667)