hydrogen - a nonmetallic univalent element that is normally a colorless and odorless highly flammable diatomic gas; the simplest and lightest and most abundant element in the universetritium - a radioactive isotope of hydrogen; atoms of tritium have three times the mass of ordinary hydrogen atoms chemical element, element - any of the more than 100 known substances (of which 92 occur naturally) that cannot be separated into simpler substances and that singly or in combination constitute all matter H2O, water - binary compound that occurs at room temperature as a clear colorless odorless tasteless liquid; freezes into ice below 0 degrees centigrade and boils above 100 degrees centigrade; widely used as a solvent gas - a fluid in the gaseous state having neither independent shape nor volume and being able to expand indefinitely |