actinium

 
Pronunciation: /akˈtɪnɪəm/

noun

[mass 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' + -ium