Real Dictionary

actinium in American English
a white, radioactive, metallic chemical element, the first member of the actinide series, found in pitchblende and other minerals or formed in reactors by the neutron irradiation of radium: symbol, Ac; at. no., 89
noun: a radioactive silver-white metallic element that glows blue in the dark, resembling the rare earths in chemical behavior and valence; at. no.: 89; at. wt.: 227

actinium in British English
noun: a radioactive element of the actinide series, occurring as a decay product of uranium. It is used as an alpha-particle source and in neutron production. Symbol: Ac; atomic no: 89; half-life of most stable isotope,227Ac: 21.6 years; relative density: 10.07