The $rad$ is a deprecated unit of absorbed radiation dose, defined as $1 rad = 0.01\,Gy = 0.01 J/kg$. It was originally defined in CGS units in 1953 as the dose causing 100 ergs of energy to be absorbed by one gram of matter. It has been replaced by the gray in most of the world. A related unit, the $roentgen$, was formerly used to quantify the number of rad deposited into a target when it was exposed to radiation. The F-factor can used to convert between rad and roentgens. The material absorbing the radiation can be human tissue or silicon microchips or any other medium (for example, air, water, lead shielding, etc.).
Rad