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

First, in most materials, the thorium nuclei release their energy not as light, but through a process called internal conversion, where the energy is transferred to an electron in the material instead. —Andrei Derevianko, The Conversation, 14 Apr. 2026

Scientists from Xinjiang University in China have allegedly developed a new ultraviolet (UV) producing crystal that could help build extremely accurate thorium nuclear clocks. —Christopher McFadden, Interesting Engineering, 12 Apr. 2026

The result suggests that heat-producing elements such as thorium may be more widely distributed beneath the lunar surface than previously thought, researchers say. —Sharmila Kuthunur, Space.com, 3 Apr. 2026

Science Minister Chang Lih Kang said Monday that any radioactive waste generated within the next five years must be treated and neutralized by extracting thorium or other methods. —ABC News, 2 Mar. 2026

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