Means giving off energy particles or waves, including heat or light (see ' electromagnetic radiation'). But often used to mean radioactive radiation, in other words gamma rays or alpha or beta particles from a radioactive source. The radioactivity comes from the breakdown of atoms. The source can be natural or made in a nuclear reactor. Uncontrolled radiation can be dangerous and cause cancer. Controlled exposure to radiation can be used in medicine for diagnosis (for example X-rays) or to treat cancer ( radiotherapy).