Noun

1. Energy that is radiated or transmitted in the form of rays or waves or particles
2. The act of spreading outward from a central source
3. 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
4. The spontaneous emission of a stream of particles or electromagnetic rays in nuclear decay
5. The spread of a group of organisms into new habitats
6. A radial arrangement of nerve fibers connecting different parts of the brain
7. (medicine) the treatment of disease (especially cancer) by exposure to a radioactive substance