[ rey-dee-oh-ak-tiv ]
/ ˌreɪ di oʊˈæk tɪv /
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Physics, Chemistry. of, relating to, exhibiting, or caused by radioactivity: A pressing issue in post-Soviet Russia is safe long-term storage of radioactive waste, from both military and civilian applications.
relating to or being a person, topic, or matter that is likely to provoke intense negative reactions or disagreement: The option of raising taxes to fund these expenditures is a politically radioactive approach that a majority of state policymakers have avoided.
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Origin of radioactive

First recorded in 1895–1900; radio- + active

OTHER WORDS FROM radioactive

ra·di·o·ac·tive·ly, adverbnon·ra·di·o·ac·tive, adjectiveun·ra·di·o·ac·tive, adjective
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2023


What does radioactive mean?

Radioactive describes something that exhibits or is caused by radioactivity. If something is radioactive, it emits radiation, which usually takes the form of electromagnetic waves or fast-moving elementary particles, such as protons or neutrons.

While low doses of radiation are usually harmless, being exposed to large amounts of radiation will most likely kill you. For this reason, highly radioactive materials are often considered very dangerous and will only be handled by experts.

Example: The radioactive waste produced by nuclear power plants can damage the environment. 

Where does radioactive come from?

Radioactive was first recorded around 1895. It combines radio-, which refers to energy transmitted in wave motion (radiant energy), and active, which describes something being in action or motion. If something is radioactive, it is releasing energy that is usually moving very fast.

What makes something radioactive? In chemistry, everything is made up of atoms, and that atom has a nucleus containing protons and neutrons. The nucleus wants the same number of electrons and neutrons. If they aren’t the same, the nucleus will remove electrons or neutrons until there is an equal number within the nucleus. The released protons and neutrons are called radiation, and an element that releases radiation is radioactive.

Radioactive materials can be dangerous because the radiation can damage a living thing’s DNA (which is really bad) or cause mutations in living cells (which is also really bad).

While the effects will be different depending on the specifics of the radioactive material, one of the most common negative effects is cancer, which is caused by mutating cells. For this reason, governments usually will not allow people (or businesses) to own or handle dangerous radioactive materials without a license.

Because of these laws, unless you work in a nuclear power plant or are a nuclear chemist, you most likely won’t have to worry about whether something is radioactive.

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What are some other forms related to radioactive?

  • radioactivity (noun)
  • radioactively (adverb)
  • nonradioactive (adjective)
  • unradioactive (adjective)

What are some words that share a root or word element with radioactive

What are some words that often get used in discussing radioactive?

How is radioactive used in real life?

In everyday life, radioactive is generally used only by scientists and people interested in nuclear science or in related news stories. However, radioactive materials sometimes appear in fiction and pop culture.

Try using radioactive!

True or False?

If something is radioactive, it is stable and will not release any radiation.

How to use radioactive in a sentence

British Dictionary definitions for radioactive

/ (ˌreɪdɪəʊˈæktɪv) /

exhibiting, using, or concerned with radioactivity

Derived forms of radioactive

radioactively, adverb
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Cultural definitions for radioactive


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

The New Dictionary of Cultural Literacy, Third Edition Copyright © 2005 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.