- an isotope of hydrogen, having twice the mass of ordinary hydrogen; heavy hydrogen. Symbols:2H, D; atomic weight: 2.01; atomic number: 1.
- a stable isotope of hydrogen, occurring in natural hydrogen (156 parts per million) and in heavy water: used as a tracer in chemistry and biology. Symbol: D or ²H; atomic no: 1; atomic wt: 2.014; boiling pt: –249.7°C
