roent·gen also rönt·gen 
(rĕntgən, -jən, rŭnt-)
(rĕntgən, -jən, rŭnt-)n. Abbr. R or r
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.
[After Wilhelm Conrad Roentgen.]
roentgen adj.