# protium

> English word · Noun · IPA /ˈpɹəʊ.tiəm/

## Definitions
1. The lightest and most common isotope of hydrogen, having a single proton and no neutrons: 11H. It is the only stable nuclide with no neutrons.
2. An atom of this isotope.

## Etymology
From proto- + -ium. From Ancient Greek πρῶτος (prôtos, “first”).

## Source
Compiled from Wiktionary via kaikki.org (CC BY-SA). Data vintage: 2026-05-06.
Canonical page: https://plainspell.com/en/word/protium