Longman Dictionary of Contemporary English - radiation

~ n 1 a form of energy that comes especially from nuclear reactions, which is very harmful to living things if present in large amounts  (an escape of low-level radiation from the nuclear power plant | lethal doses of radiation) 2 energy in the form of heat or light sent out as beams that you cannot see  (electromagnetic radiation | sun cream that filters out harmful ultraviolet radiation) 3 the radiating of heat, light etc