Protium

Japanese: protium

…However, the stable and widely occurring ^2H is usually called deuterium (chemical symbol D), and the unstable radioactive element ^3H , which exists only in small amounts in nature, is called tritium (chemical symbol T) to distinguish it from deuterium. In contrast, ordinary hydrogen ^1H is also called protium. The nucleus of D, D ⁺ (consisting of one proton and one neutron), is called a deuteron, and the nucleus of T, T ⁺ (consisting of one proton and two neutrons), is called a triton, to distinguish it from H ^+, i.e., a proton. …

From [Hydrogen]

...The radioisotope with mass number 3 ( ^3H ) is called tritium or tritium, but ^2H and ^3H are sometimes collectively called deuterium. In contrast, ^1H is sometimes called protium or light hydrogen. Tritium decays by emitting beta rays. ...

*Some of the terminology explanations that mention "protium" are listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

Japanese:

…しかし通常はそのうち安定で広く存在する²Hを重水素あるいはジュウテリウムdeuterium(化学記号D)と呼び，不安定な放射性元素で天然に微量しか存在しない³Hは三重水素あるいはトリチウムtritium(化学記号T)と呼んでこれと区別することがある。またこれらに対し，ふつうの水素¹Hを軽水素あるいはプロチウムprotiumとも呼ぶ。またDの原子核D^＋(1個の陽子と1個の中性子から成る)を重陽子あるいはジュウテロンdeuteron,Tの原子核T^＋(1個の陽子と2個の中性子から成る)を三重陽子あるいはトリトンtritonと呼び，H^＋すなわち陽子(プロトンproton)と区別する。…