An inert, monatomic, colorless, odorless element, the lightest of the rare gases. Except for hydrogen, helium is the most abundant element found in the universe. The bulk of the world's supply is obtained from wells. Symbol, He. Widely used in cryogenic research; vital in the study of superconductivity. Helium is used for arc welding, as a cooling medium for nuclear reactors, and as a gas for supersonic wind tunnels; extensively used for filling balloons as it is much safer than hydrogen. One of the recent largest uses for helium has been for pressuring liquid fuel rockets.