/ ˈrɜːntjən; ˈrɒnt-; -gən; NAmE ˈrentjən; ˈrənt-; -dʒən/ noun
( physics ) ( abbr. R ) a unit of radiation , used to measure the quantity of X-rays or gamma rays that have reached sb/sth
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WORD ORIGIN
1920s: named after Wilhelm Conrad Röntgen (1845–1923), German physicist, discoverer of X-rays.