The distance into the sample measured from the physical surface from which all but a fraction 1/e of the particles or radiation detected have originated.
Source:
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)