According to the International Atomic Energy Agency(IAEA), an accident is any unintended event, including operating errors, equipment failures or other mishaps, the consequences or potential consequences of which are not negligible from the point of view of protection or safety. Source: IAEA (Retrieved Jan. 9, 2015).
The property possessed by some elements (such as uranium) or isotopes (such as carbon 14) of spontaneously emitting energetic particles (such as electrons or alpha particles) by the disintegration of their atomic nuclei; also : the rays emitted (Definition from Merriam/Webster)
A radioactive isotope, usually artificially produced: used in physical and biological research, therapeutics, etc. (From Dictionary.com)