This term was named after Wilhelm Konrad Roentgen who discovered X-rays and X-Ray photography.
Laymans Definition: For radiation protection purposes, an exposure to 1 roentgen of gamma rays is generally assumed to produce a dose of 1 rad in water or soft tissue.
Dictionary: The word refers to a unit of exposure, based on a amount of ionisation (charge) produced by the absorption of gamma radiation energy in air under standard conditions.
Scientific Definition: A unit of radiation exposure equal to the quantity of ionizing radiation that will produce one electrostatic unit of electricity in one cubic centimeter of dry air at 0°C and standard atmospheric pressure.
Formula: 1R = 2.58 x 10-4 C kg-1 of air