Meaning of RADIOACTIVITY in English
/ray'dee oh ak tiv"i tee/ , n. Physics, Chem.
the phenomenon, exhibited by and being a property of certain elements, of spontaneously emitting radiation resulting from changes in the nuclei of atoms of the element. Also called activity .
[ 1895-1900; RADIO- + ACTIVITY ]
Random House Webster's Unabridged English dictionary. Полный английский словарь Вебстер - Random House . 2012