noun
US/ˌreɪ.di.oʊ.ækˈtɪv.ə.ti/
UK/ˌreɪ.di.əʊ.ækˈtɪv.ɪ.ti/
The dangerous energy that some materials send out as tiny invisible particles.
In context
Scientists measured the radioactivity of the water near the old power plant.
Short entry. Full notes coming.