energy in a wide variety of forms that makes up the electromagnetic spectrum. It ranges from light and heat to radio waves, microwaves, and x-rays. In medicine, the 2 main types are ionizing and non-ionizing radiation. The high-energy rays that are used for x-rays, and in higher doses for cancer treatment, are called ionizing radiation. Ionizing radiation has enough energy to remove electrons from atoms. This type of radiation can be produced by medical devices, but also comes from natural sources such as radon gas (in the ground) and outer space. See also radiation dose, radiation therapy, x-ray.