ra ‧ di ‧ um /ˈreɪdiəm/ BrE AmE noun [uncountable]
[ Date: 1800-1900 ; Language: Modern Latin ; Origin: Latin radius ; ⇨ ↑ radius ]
a white metal that is ↑ radioactive and is used in the treatment of diseases such as ↑ cancer . It is a chemical ↑ element : symbol Ra