1. A photon of electromagnetic radiation of very short wavelength, less than about 0.01 nanometer, and very high energy, greater than about 100,000 electron volts. Gamma rays are emitted in the decay of certain radioactive nuclei and in electron-positron annihilation.
2. A narrow beam of such photons.
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2020 by Houghton Mifflin Harcourt Publishing Company. 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.