a. A part or particle considered to be an irreducible constituent of a specified system.
b. The irreducible, indestructible material unit postulated by ancient atomism.
2. An extremely small part, quantity, or amount.
3. Physics & Chemistry
a. The smallest unit of an element, having all the characteristics of that element and consisting of a very small and dense central nucleus containing protons and neutrons, surrounded by one or more shells of orbiting electrons. Atoms remain undivided in chemical reactions except for the donation, acceptance, or exchange of valence electrons.
b. This unit regarded as a source of nuclear energy.
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2018 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.