Radon is a gas and a chemical element (symbol Rn). It is radioactive: it gives out radiation by itself. You cannot see it or smell it. Older learner books often say it is used to treat cancer. That was true in a small way — some clinics used it, and a few spa towns still sell radon baths. Though actually, in ordinary life now the word is mostly a warning, not a treatment. Radon can seep up from rock and soil into a house, especially a poorly aired basement, and breathing a lot of it for years is bad for the lungs. It is a science and news word.