Is it farther or further?
1898, from French radio-actif, coined by Pierre and Marie Curie from radio-, comb. form of Latin radius (see radiation) + actif "active" (see active).
radioactive ra·di·o·ac·tive (rā'dē-ō-āk'tĭv)adj. Of or exhibiting radioactivity.
A descriptive term for a material made up of atoms in which radioactivity occurs.