rad, the unit of absorbed dose of ionizing radiation, defined in 1962 by the International Commission on Radiological Units and Measurements as equal to the amount of radiation that releases an energy of 100 ergs per gram of matter. One rad is equal approximately to the absorbed dose delivered when soft tissue is exposed to one roentgen of medium-voltage radiation. “Rad” is derived from “radiation absorbed dose.” In 1975 it was replaced by the gray (Gy), equal to 100 rads, in the International System of Units (SI). The rad is used now only in the United States.