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Radioactivity: The spontaneous emission of radiation, generally alpha or beta particles, often accompanied by gamma rays, from the nucleus of an unstable isotope. Also, the rate at which radioactive material emits radiation. Measured in units of becquerels or disintegrations per second.

Image Source: U.S. Nuclear Regulatory Commission

http://www.nrc.gov/reading-rm/basic-ref/glossary/full-text.html