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

Operating since 1997, the reactor used deuterium and tritium as fuel and achieved record-breaking energy outputs during its nearly 25 years of operation. —Ameya Paleja, Interesting Engineering, 27 Jan. 2026

That's Gr5 titanium to be exact, and the slim playing edge narrows to a curved point, while the chunkier grip portion features anti-slip grooves plus slots for optional tritium tubes – for a little extra glow-in-the-dark pizzazz. —New Atlas, 19 Jan. 2026

One way to explain this is to actually take from the history over at Peach Bottom, toward the east, where ‘tritium leaks’ have been a source of tension since 1977. —John Werner, Forbes.com, 17 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

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