Definition of actinium noun from the Oxford Advanced Learner's Dictionary

 

actinium

 noun
noun
BrE BrE//ækˈtɪniəm//
 
; NAmE NAmE//ækˈtɪniəm//
 
[uncountable](symbol Ac)
 
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a chemical element. Actinium is a radioactive metal. Word Origin early 20th cent.: from Greek aktis, aktin- ‘ray’ + -ium, used to form names of metallic elements.
See the Oxford Advanced American Dictionary entry: actinium