: 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 WebOther 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
Word History
Etymology
borrowed from French, from Greek aktīn-, aktís "ray, beam" + -ium-ium; so called from its radioactivity — more at actin-