Diameter of the beam between which 68% of the electrons fall. For Gaussian beam shapes this corresponds to the 2σ-value of the intensity distribution of the beam. Measurement can be carried out by scanning a beam across a sufficiently sharp edge and recording the transmitted electrons.
PAC, 1983, 55, 2023. 'Nomenclature, symbols and units recommended for in situ microanalysis (Provisional)' on page 2027 (https://doi.org/10.1351/pac198355122023)