radioactivityra‧di‧o‧ac‧tiv‧i‧ty /ˌreɪdiəʊækˈtɪvəti, ˌreɪdiəʊækˈtɪvɪti $ -dioʊ-/
noun [uncountable]the sending out of
radiation (=a form of energy) when the
nucleus (=central part) of an atom has broken apart:
the discovery of radioactivity high levels of radioactivity