https://doi.org/10.1351/goldbook.ET02276
For @X06703@ or @R02568@ in air: the sum of the electrical charges of all the @I03158@ of one sign produced when all @E01975@s liberated by @P04627@s in a suitably small element of volume of air completely stopped, divided by the mass of the air in the volume element.
This definition replaces an earlier definition of exposure.