The absorbed dose D is the quotient of the average energy transferred to the matter in a volume element by ionizing radiation and the mass of the matter in this volume element:

The unit of the absorbed dose is joule divided by kilogram (J·kg-1) and its special unit name is gray (Gy). The former unit name was rad (symbol: rd or rad).1 Gy = 100 rd; 1 rd = 1/100 Gy.