Definition of "hydrogen" in English

noun

1. The lightest chemical element (symbol H), with an atomic number of 1 and atomic weight of 1.008.

2. Molecular hydrogen (H₂), a colourless, odourless and flammable gas at room temperature.

3. The isotope hydrogen-1 (protium) (symbol H or ¹H), contrasting with deuterium and tritium

   • Hydrogen is generally considered to be electronically the same as deuterium. […] In both PdHₓ and PdDₓ […] a resistivity maximum is found near 50 K.

4. A sample of the element/molecule.