tritium

noun

tri·​ti·​um ˈtri-tē-əm  ˈtri-shē- 

: a radioactive isotope of hydrogen that has one proton and two neutrons in its nucleus and that has three times the mass of ordinary hydrogen

—symbol T

Examples of tritium in a Sentence

Recent Examples on the Web

Katrin uses its house-size detector to look for electrons released during radioactive decays of tritium atoms. —Quanta Magazine, 8 Apr. 2026

The spill comes more than three years after 400,000 gallons of water containing the radioactive isotope tritium leaked into groundwater underneath the facility. —Wcco Staff, CBS News, 11 Mar. 2026

Optional glow-in-the-dark tritium tubes can be added to the sides of the knife for an extra fee. —Maryna Holovnova, New Atlas, 27 Feb. 2026

Helion was the first company licensed to use radioactive tritium as a fusion source. —Jordan Blum, Fortune, 13 Feb. 2026

Word History

Etymology

New Latin, from Greek tritos third — more at third entry 1

First Known Use

1933, in the meaning defined above

Kids Definition

tritium

noun

tri·​ti·​um ˈtrit-ē-əm  ˈtrish-ē- 

: a rare radioactive form of hydrogen having atoms with three times the mass of ordinary light hydrogen atoms

Medical Definition

tritium

noun

tri·​ti·​um ˈtrit-ē-əm  ˈtrish-ē- 

: a radioactive isotope of hydrogen that has three times the mass of ordinary hydrogen

—symbol T