A unit of measure of absorbed radiation is known as the rad. It measures absorbed dose: the amount of ionizing-radiation energy deposited in a given mass of material, such as human tissue, air, or water.
One rad equals 0.01 gray (Gy), or 0.01 joule per kilogram. The name comes from “radiation absorbed dose,” and the unit was defined in the CGS measurement system. It was introduced in 1953 to describe a dose depositing 100 ergs of energy in one gram of matter.
The rad is not the modern SI unit; the gray replaced it as the preferred international unit. The rad remains familiar in older literature and is still encountered in some United States contexts. In medical radiation therapy, the numerically equivalent centigray is commonly used.
A frequent mix-up is confusing absorbed dose with exposure or biological effect. The roentgen measures exposure in air, while the rem and sievert account more directly for biological weighting and radiation risk. The rad describes energy absorbed, not the complete health impact.