1. 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 universe. atomic number 1, H. tritium-a radioactive isotope of hydrogen; atoms of tritium have three times the mass of ordinary hydrogen atoms.ymbol H A colorless, odorless, highly flammable gaseous element that is the lightest and most abundant element in the universe. It occurs in water in combination with oxygen, in most organic compounds, and in small amounts in the atmosphere as a gaseous mixture of its three isotopes (protium, deuterium, and tritium).