D defs.my
Entry 1 sense Webster, 1913

Actinium

/ăktĭn'ēəm/ · Ac·tin·i·um · IPA /ækˈtɪniəm/
01 n. A supposed metal, said by Phipson to be contained in commercial zinc; -- so called because certain of its compounds are darkened by exposur…
  1. 1.
    A supposed metal, said by Phipson to be contained in commercial zinc; -- so called because certain of its compounds are darkened by exposure to light.(Chem.)