radioactivity

Definition: The spontaneous conversion of certain atomic nuclei into those of lighter elements accompanied by the emission of alpha particles (helium nuclei), beta particles (electrons or positrons), or X and gamma radiation (short wavelength/high frequency electromagnetic waves). Discovered in 1896 by Becquerel when a smaple of uranium was left on top of (and exposed) photographic plates by means of the x-rays it naturally emitted.