noun.
A colorless, radioactive, inert gaseous element formed by the radioactive decay of radium, that is used in radiotherapy and to produce neutrons for research. Its most stable isotope is Rn-222 with a half-life of 3.82 days. A natural source of radiation found in most soils and groundwater, radon poses a serious health threat if inhaled. Atomic number 86; melting point −71°C; boiling point −61.7°C; density of gas 9.73 grams per liter; specific gravity (solid) 4. cross-reference: Periodic Table.