For X- or gamma-radiation in air: the sum of the electrical charges of all the ions of one sign produced when all electronss liberated by photonss in a suitably small element of volume of air completely stopped, divided by the mass of the air in the volume element.
Source:
PAC, 1982, 54, 1533. 'Glossary of terms used in nuclear analytical chemistry (Provisional)' on page 1541 (https://doi.org/10.1351/pac198254081533)