The term 'Radiation Unit' can be broken down into four parts: REM (3 letters), Sievert (7 letters), RAD (3 letters), and Roentgen (8 letters). REM stands for 'Roentgen Equivalent Man, ' which is a unit of radiation dose equivalent to the biological effect of one roentgen of X-rays or gamma rays. Sievert is a unit of radiation dose equivalent to the biological effect of one joule of energy per kilogram of tissue. RAD stands for 'Radiation Absorbed Dose, ' which is a unit of absorbed radiation dose, equal to 100 ergs per gram of irradiated material. Finally, Roentgen is a unit of exposure to ionizing radiation, equal to the amount of gamma radiation that will produce one electrostatic unit of charge in one cubic centimeter of dry air.