ˌɹeɪ.di.ˈeɪ.ʃən (medicine) the treatment of disease (especially cancer) by exposure to a radioactive substance the act of spreading outward from a central source a radial arrangement of nerve fibers connecting different parts of the brain the spread of a group of organisms into new habitats energy that is radiated or transmitted in the form of rays or waves or particles the spontaneous emission of a stream of particles or electromagnetic rays in nuclear decay syndrome resulting from exposure to ionizing radiation (e.g., exposure to radioactive chemicals or to nuclear explosions); low doses cause diarrhea and nausea and vomiting and sometimes loss of hair; greater exposure can cause sterility and cataracts and some forms of cancer and other diseases; severe exposure can cause death within hours he was suffering from radiation