actinium Syllabification: (ac·tin·i·um)Pronunciation: /akˈtinēəm/Definition of actinium noun the chemical element of atomic number 89, a radioactive metallic element of the actinide series. It is rare in nature, occurring as an impurity in uranium ores. (Symbol: Ac)Origin: early 20th century: from Greek aktis, aktin- 'ray' + -iumactinium in other Oxford dictionaries Definition of actinium in the British & World English dictionary
