In science, radiation describes both all electromagnetic phenomena - photons created when atoms absorb energy and re-emit it, and fast electrons, such as those in cosmic rays and those created by the decay of heavy elements.
However, in medicine, it usually refers only to ionizing radiation, those forms of radiation that can cause severe damage to human tissue - ultraviolet radiation, gamma rays, X-rays, and beta radiation. In fact, most forms of radiation, from radio waves to visible light, pose no risk to human health.
Radiation is the key element of the medical speciality of radiology - the use of radiation to diagnose and treat disease.