radiation

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The act or process of radiating.
• noun Emission or propagation of energy in the form of waves or particles.
• noun Energy radiated or transmitted in the form of waves or particles.
• noun A stream of particles or electromagnetic waves emitted by the atoms and molecules of a radioactive substance as a result of nuclear decay.
• noun The act of exposing or the condition of being exposed to such energy.
• noun The application of such energy, as in medical treatment.
• noun Anatomy Radial arrangement of parts, as of a group of nerve fibers connecting different areas of the brain.
• noun Adaptive radiation

from The Century Dictionary.

• noun The act of radiating, or the state of being radiated; specifically, emission and diffusion of rays of light and the so-called rays of heat.
• noun The divergence or shooting forth of rays from a point or focus.
• noun In zoology, the structural character of a radiate; the radiate condition, quality, or type; the radiate arrangement of parts. Also radiism.
• noun In biology: The divergent evolution of several different organisms from a single ancestral form: as, the radiation of the placental mammals.
• noun A group of organisms that is undergoing divergent modification.
• noun In psychology, the extension of excitation within the nervous system to give rise to concomitant or secondary sensations.

from the GNU version of the Collaborative International Dictionary of English.

• noun The act of radiating, or the state of being radiated; emission and diffusion of rays of light; beamy brightness.
• noun The shooting forth of anything from a point or surface, like the diverging rays of light.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The shooting forth of anything from a point or surface, like the diverging rays of light; as, the radiation of heat.
• noun The process of radiating waves or particles.
• noun The transfer of energy via radiation (as opposed to convection or conduction)
• noun Radioactive energy

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a radial arrangement of nerve fibers connecting different parts of the brain
• noun the spread of a group of organisms into new habitats
• noun 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
• noun the act of spreading outward from a central source
• noun energy that is radiated or transmitted in the form of rays or waves or particles
• noun the spontaneous emission of a stream of particles or electromagnetic rays in nuclear decay
• noun (medicine) the treatment of disease (especially cancer) by exposure to a radioactive substance