1. The act or process of radiating: the radiation of heat and light from a fire.
a. Emission or propagation of energy in the form of waves or particles.
b. Energy radiated or transmitted in the form of waves or particles.
c. A stream of particles or electromagnetic waves emitted by the atoms and molecules of a radioactive substance as a result of nuclear decay.
a. The act of exposing or the condition of being exposed to such energy.
b. The application of such energy, as in medical treatment.
4. Anatomy Radial arrangement of parts, as of a group of nerve fibers connecting different areas of the brain.
5. Adaptive radiation
ra′di·ation·al, radi·a′tive adj.
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2022 by HarperCollins Publishers. All rights reserved.
Indo-European & Semitic Roots Appendices
Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:
The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.