Definition of radium
: an intensely radioactive metallic chemical element that occurs in combination in minute quantities in minerals (such as pitchblende or carnotite), emits alpha particles and gamma rays to form radon, and is used chiefly in the treatment of cancer and in radiographic devices — see Chemical Elements Table
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Merriam-Webster unabridged
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Time Traveler for radium
The first known use of radium was in 1899
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