A unit of radiation dose equal to a millionth of a gray or 0.1 millirad. Small doses of this size are often provided by natural sources in the environment
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}