radioactive

adjective

ra·​dio·​ac·​tive ˌrā-dē-ō-ˈak-tiv How to pronounce radioactive (audio)
1
: of, caused by, or exhibiting radioactivity
radioactive isotopes
Radon is an odorless, colorless gas that arises naturally from the ground because of the decay of radioactive elements commonly found in rocks and many types of soil. In a chain of radioactive decay, uranium produces radium, which gives off radon, which in turn produces radioactive breakdown products that are harmful if inhaled.Warren E. Leary
2
: so divisive or controversial as to require avoidance
He has been deemed radioactive by most charitable organizations … and organized competition, even local stuff, is largely closed off to him.Asher Price
Almost all women—and therefore men—use a form of birth control at some point in their lives, yet contraception is so politically and legally radioactive that legislators and pharmaceutical companies avoid funding it.Karen Weise
Subsequent polling data show that the quota issue is perhaps the most powerful one in our politics … A House Democratic aide says, "The quota issue is radioactive." The result has been nothing less than panic in the Democratic ranks.Elizabeth Drew
radioactively adverb
Plutonium is the only one of these transuranic elements that can exist a fair amount of time before radioactively decaying into lighter elements. James Kaler

Examples of radioactive in a Sentence

Uranium and plutonium are radioactive.
Recent Examples on the Web
Examples are automatically compiled from online sources to show current usage. Read More Opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback.
The imaging technique relies on the use of a radioactive tracer and a special camera to make pictures of breast tissue. Hannah Millington, MSNBC Newsweek, 3 Oct. 2025 The potential promise of fusion is for consistent, clean power without radioactive waste, intermittency issues, or the dependence on foreign supply chains. Jordan Blum, Fortune, 2 Oct. 2025 These radioisotopes spontaneously transform into different elements and release radiation – energetic particles or light – in a process called radioactive decay. Andrea Richard, The Conversation, 2 Oct. 2025 Then, Rutherford set up a radioactive source on one side of the foil, surrounding the rest of the apparatus with an absorptive ring of solid material. Ethan Siegel, Big Think, 30 Sep. 2025 See All Example Sentences for radioactive

Word History

Etymology

International Scientific Vocabulary

First Known Use

1898, in the meaning defined at sense 1

Time Traveler
The first known use of radioactive was in 1898

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Cite this Entry

“Radioactive.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/radioactive. Accessed 8 Oct. 2025.

Kids Definition

radioactive

adjective
ra·​dio·​ac·​tive ˌrād-ē-ō-ˈak-tiv How to pronounce radioactive (audio)
: of, caused by, or exhibiting radioactivity
radioactively adverb

Medical Definition

radioactive

adjective
ra·​dio·​ac·​tive ˌrād-ē-ō-ˈak-tiv How to pronounce radioactive (audio)
: of, caused by, or exhibiting radioactivity
radioactive isotopes
radioactively adverb

More from Merriam-Webster on radioactive

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