A radioactive gaseous element formed by the disintegration of radium; the heaviest of the inert gasses; occurs naturally (especially in areas over granite) and is considered a hazard to health WordNet 3.0 & Open English WordNet

Radon is a chemical element with the symbol Rn and atomic number 86. Wikipedia

Chemical element with symbol Rn and atomic number 86. Wikipedia Disambiguation

A radioactive gaseous element emitted naturally from rocks and minerals where radioactive elements are present. It is released in non-coal mines, e.g. tin, iron, fluorspar, uranium. Radon is an alpha particle emitter as are its decay products and has been indicted as a cause of excessive occurrence of lung cancer in uranium miners. Concern has been expressed at radon levels in some housing usually adjacent to granite rocks or old tin mining regions. OmegaWiki

The chemical element (symbol Rn, formerly Ro) with atomic number 86. It is an odorless, colorless, chemically inert but radioactive noble gas. Wiktionary

Chemical element. Wiktionary (translation)