(chemistry) hydrogen peroxide

(physics, astronomy) a spectral line of atomic hydrogen having a wavelength of about 21cm; used extensively in radio astronomy

(chemistry) the sulfur analogue of hydrogen peroxide, H₂S₂ / (erroneously) hydrogen sulfide

(chemistry, physics) an electrode in which hydrogen gas is adsorbed on platinum

(informal) dihydrogen monoxide (water (H₂O), but not hydrogen peroxide (H₂O₂) or the hydroxyl radical (-OH))

(inorganic chemistry, chiefly Britain) Alternative spelling of hydrogen sulfide

(chemistry) the hydride of selenium, H₂Se; it is a toxic, inflammable gas, having properties similar to those of hydrogen sulfide

(inorganic chemistry) An inorganic compound with the formula HCl. Forms hydrochloric acid when dissolved in water.

(inorganic chemistry, countable) Synonym of bicarbonate / (inorganic chemistry, uncountable) carbonic acid

(physics) all the lines in the emission or absorption spectrum of hydrogen; each one corresponding to an allowed transition between quantum energy levels