adjective Physics, Chemistry.
of, pertaining to, exhibiting, or caused by radioactivity.

1895–1900; radio- + active

radioactively, adverb
nonradioactive, adjective
unradioactive, adjective Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
radioactive (ˌreɪdɪəʊˈæktɪv)
exhibiting, using, or concerned with radioactivity

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

1898, from Fr. radio-actif, coined by Pierre and Marie Curie from radio-, comb. form of L. radius (see radiation) + actif (fem. active) "active."
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

radioactive ra·di·o·ac·tive (rā'dē-ō-āk'tĭv)
Of or exhibiting radioactivity.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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American Heritage
Cultural Dictionary

radioactive definition

A descriptive term for a material made up of atoms in which radioactivity occurs.

The American Heritage® New Dictionary of Cultural Literacy, Third Edition
Copyright © 2005 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
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Example sentences
The raindrops that keep falling on our heads may not cause our hair to fall out
  but they are, nevertheless, radioactive.
It would also not itself become radioactive by absorbing neutrons.
Around the world radioactive nuclear reactors in derelict submarines dot the
Radioactive sulphur signal adds to evidence of catastrophe.
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