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Definition of: radioactivity
Our photo definition of radioactivity 
(rā′dē·ō·ak·tiv′ə·tē) noun Physics
1. The propagation of radiant energy.
2. The spontaneous nuclear disintegration of certain elements and isotopes, with the emission of alpha particles, electrons, positrons, or electromagnetic radiation.
3. A particular form of such disintegration: gamma radioactivity.
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