Real Dictionary

radon
uncountable noun: Radon is a radioactive element in the form of a gas.

radon in American English
a radioactive, gaseous chemical element, one of the noble gases, formed, together with alpha rays, as a first product in the atomic disintegration of radium: symbol, Rn; at. no., 86
noun: a chemically inert, radioactive gaseous element produced by the decay of radium: emissions produced by outgassing of rock, brick, etc. are a health hazard; at. no.: 86; at. wt.: 222

radon in British English
noun: a colourless radioactive element of the rare gas group, the most stable isotope of which, radon-222, is a decay product of radium. It is used as an alpha particle source in radiotherapy. Symbol: Rn; atomic no: 86; half-life of 222Rn: 3.82 days; valency: 0