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

There are also add-ons on offer, including a tritium tube that doesn't need to be charged, a cheaper glass luminous tube, replacement blades, and a quick release keychain. —Abhimanyu Ghoshal, New Atlas, 9 Jan. 2026

For deuterium-tritium fusion reactions, plasmas must be heated to an optimal temperature of around 13 keV (150 million kelvin). —Prabhat Ranjan Mishra, Interesting Engineering, 2 Jan. 2026

The first-ever project to demonstrate the deuterium-tritium fuel cycle—the most viable route to practical fusion energy—plans to be operational by late 2026. —IEEE Spectrum, 31 Dec. 2025

Most experts agree that significant technical hurdles in plasma confinement, tritium fuel cycle management, and first-wall materials must be overcome before commercial fusion becomes reality. —Dave Smith, Fortune, 18 Dec. 2025

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