The distance between the 84 and 16 per cent levels of the depth profile of an element in a perfect sandwich sample with an infinitesimally small overlap of the components. These limits correspond to the 2σ-value of the Gaussian distribution of the signal at the interface.
PAC, 1979, 51, 2243. 'General aspects of trace analytical methods—IV. Recommendations for nomenclature, standard procedures and reporting of experimental data for surface analysis techniques' on page 2247 (https://doi.org/10.1351/pac197951112243)