ra ‧ di ‧ o ‧ ac ‧ tiv ‧ i ‧ ty /ˌreɪdiəʊækˈtɪvəti, ˌreɪdiəʊækˈtɪvɪti $ -dioʊ-/ BrE AmE 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