heavy hydrogen(n = noun.substance) deuterium - an isotope of hydrogen which has one neutron (as opposed to zero neutrons in hydrogen);
is a kind of isotope, hydrogen atom

acid hydrogen, acidic hydrogen, heavy, heavy cream, heavy hand, heavy handed, heavy heart, heavy hitter, heavy lifting, heavy metal, heavy particle, heavy spar, heavy swell, heavy water, heavy weapon, heavy whipping cream, hydrogen, hydrogen atom, hydrogen azide, hydrogen bomb, hydrogen bond, hydrogen bromide, hydrogen carbonate, hydrogen chloride, hydrogen cyanide, hydrogen fluoride, hydrogen iodide, hydrogen ion, hydrogen ion concentration, hydrogen peroxide, hydrogen sulfide, potassium hydrogen carbonate, potassium hydrogen tartrate, sodium hydrogen carbonate, top heavy