What is "hydrium"

n. 1 (context obsolete chemistry English) ''a metallic gas'' — a former name of hydrogen ''Fireside Science'', , 1872 2 (context rare chemistry English) protium "Alkylation of Alkanes", by ''Gustav Egloff'' & ''George Hulla'', published by '''Reinhold Pub. Corp.''', '1948'

Hydrium may refer to several things:

• Hydrogen, hydrium is a former name for hydrogen.
• Hydrium, a fictitious element in Kenneth Oppel's Matt Cruse novels, Airborn, Skybreaker, and Starclimber