radioactive

adjective

ra·​dio·​ac·​tive ˌrā-dē-ō-ˈak-tiv 

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

The PET scan uses a radioactive drug called a tracer to show both typical and atypical metabolic activity, according to the Mayo Clinic. —Stephanie Giang-Paunon, FOXNews.com, 18 Feb. 2026

In 2006, Russian defector Alexander Litvinenko died after drinking a cup of tea laced with radioactive polonium-210 in a London hotel. —The Week Uk, TheWeek, 17 Feb. 2026

The allegation feeds into a long and deeply contested narrative around high-profile poison cases in Vladimir Putin’s Russia, from radioactive tea to nerve agents. —Freddie Clayton, NBC news, 15 Feb. 2026

Russia also denied poisoning Alexander Litvinenko, a former Russian agent turned Kremlin critic who died in London in 2006 after ingesting the radioactive isotope polonium-210. —Jill Lawless, Los Angeles Times, 14 Feb. 2026

Word History

Etymology

International Scientific Vocabulary

First Known Use

1898, in the meaning defined at sense 1

Rhymes for radioactive

• bioactive
• hyperactive
• interactive
• nonreactive
• overactive
• psychoactive
• putrefactive
• retroactive
• unattractive
• unreactive
• vasoactive
• active

Kids Definition

radioactive

adjective

ra·​dio·​ac·​tive ˌrād-ē-ō-ˈak-tiv 

: of, caused by, or exhibiting radioactivity

radioactively adverb

Medical Definition

radioactive

adjective

ra·​dio·​ac·​tive ˌrād-ē-ō-ˈak-tiv 

: of, caused by, or exhibiting radioactivity

radioactive isotopes

radioactively adverb