The amount of ionizing radiation required to produce the same biological effect as one rad of high-penetration x-rays.
A unit for measuring absorbed doses of radiation, equivalent to one roentgen of x-rays or gamma rays.
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.