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at·om (ătəm)
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n.
1.
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.

[Middle English attome, from Latin atomus, from Greek atomos, indivisible, atom : a-, not; see A-1 + tomos, cutting (from temnein, to cut; see tem- in the Appendix of Indo-European roots).]

The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2017 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
 

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