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actinium (n.)

radioactive element discovered in 1899; see actino- "pertaining to rays" + chemical suffix -ium. It emits beta rays. The name was given earlier to a supposed new element (1881).

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Definitions of actinium from WordNet

actinium (n.)
a radioactive element of the actinide series; found in uranium ores;
Synonyms: Ac / atomic number "
From wordnet.princeton.edu