- noun a colourless poisonous strong-smelling gas.
- A colorless, corrosive, and poisonous gas whose chemical formula is HF. It is used in plasma etching, and in lasers. hydrogen half cell Same as hydrogen electrode. hydrogen ion A hydrogen atom which has been stripped of its electron. In the case of the protium isotope, which is by far the most common, this means leaving just a proton. Its symbol is H+.