actinium
noun
ac·tin·i·um ak-ˈti-nē-əm 
: a radioactive trivalent metallic element that resembles lanthanum in chemical properties and that is found especially in pitchblende see Chemical Elements Table
Example Sentences
Recent Examples on the Web Other theorists have predicted that hydrides such as calcium hydride or actinium hydride should superconduct at close to room temperature—and at a pressure considerably less than that needed for CSH. 
Robert F. Service, Science | AAAS, 26 Aug. 2021
Robert F. Service, Science | AAAS, 26 Aug. 2021 Word History
Etymology
borrowed from French, from Greek aktīn-, aktís "ray, beam" + -ium -ium; so called from its radioactivity — more at actin-
First Known Use
1900, in the meaning defined above
Medical Definition
actinium
noun
ac·tin·i·um ak-ˈtin-ē-əm
: a radioactive trivalent metallic element that resembles lanthanum in chemical properties and that is found especially in pitchblende —symbol Ac see Chemical Elements Table