Yours, Etc.: Origins and Uses of 8 Sign-Offs
protium pro·ti·um (prō'tē-əm, prō'shē-)
The most abundant isotope of hydrogen, having an atomic mass of 1. Its nucleus consists of a single proton. See more at hydrogen.
isotope of hydrogen (q.v.) with atomic weight of approximately 1; its nucleus consists of only one proton. Ordinary hydrogen is made up almost entirely of protium.