A unit of radiation dose equal to 10-6 sievert or 0.1 millirem. The radiation doses resulting from exposure to natural sources such as radon gas in the atmosphere are often measured in this unit
Russ Rowlett (Director), How Many? A Dictionary of Units of Measurement, Center for Mathematics and Science Education University of North Carolina, USA 2005. http://www.unc.edu/~rowlett/units/dictA.html, (Unit of Meas and Covr Factors - Amer) {608}