thorium

noun

tho·​ri·​um ˈthȯr-ē-əm 

: a radioactive metallic element that is obtained especially from monazite and is usually associated with rare earths see Chemical Elements Table

Examples of thorium in a Sentence

Recent Examples on the Web

Certain types of rare earth ores also contain radioactive thorium or uranium, which is often removed using acid. —Jason Neely and Eric Onstad, USA TODAY, 27 Feb. 2025

Today many astronomical spectrographs are calibrated using the spectral lines emitted by thorium argon lamps, which have irregular lines that vary widely in brightness and spacing and shine on only a small fraction of the CCD cells in a detector. —IEEE Spectrum, 30 Dec. 2011

These were accompanied by supposed paper records confirming it to be thorium and uranium. —Andrew Paul, Popular Science, 9 Jan. 2025

In this kind of reactor, thorium fluoride dissolves in molten salt in the reactor’s core. —Emily Waltz, IEEE Spectrum, 30 Dec. 2024

Word History

Etymology

New Latin, from Old Norse Thōrr Thor

First Known Use

1832, in the meaning defined above

Rhymes for thorium

• fusarium
• solarium
• terrarium

Kids Definition

thorium

noun

tho·​ri·​um ˈthōr-ē-əm 

ˈthȯr-

: a radioactive metallic element see element

Medical Definition

thorium

noun

tho·​ri·​um ˈthōr-ē-əm, ˈthȯr- 

: a radioactive metallic element that occurs combined in minerals and is usually associated with rare earths

—symbol Th see Chemical Elements Table