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[at-uh m] /ˈæt əm/
  1. the smallest component of an element having the chemical properties of the element, consisting of a nucleus containing combinations of neutrons and protons and one or more electrons bound to the nucleus by electrical attraction; the number of protons determines the identity of the element.
  2. an atom with one of the electrons replaced by some other particle:
    muonic atom; kaonic atom.
Energy. this component as the source of nuclear energy.
a hypothetical particle of matter so minute as to admit of no division.
anything extremely small; a minute quantity.
Origin of atom
1350-1400; Middle English attomos, athomus < Latin atomus < Greek átomos, noun use of átomos undivided, equivalent to a- a-6 + tomós divided, verbid of témnein to cut
4. shred, speck, scintilla, iota, jot, whit. Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2018.
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Examples from the Web for atom
Contemporary Examples
Historical Examples
  • The ion differs from the molecule, the corpuscle and the atom in that it is an ion.

    The Devil's Dictionary Ambrose Bierce
  • Fictions or realities, could they survive the touchstone of this atom of common sense?

    The Uncommercial Traveller Charles Dickens
  • It wad but raise a strife atween the twa, ohn dune an atom o' guid.

    Heather and Snow George MacDonald
  • Even the good things that the atom had brought were destroyed in the panic that ensued.

    Now We Are Three Joe L. Hensley
  • The atom retains its identity through all combinations and processes.

    The Machinery of the Universe Amos Emerson Dolbear
British Dictionary definitions for atom


  1. the smallest quantity of an element that can take part in a chemical reaction
  2. this entity as a source of nuclear energy: the power of the atom See also atomic structure
any entity regarded as the indivisible building block of a theory
the hypothetical indivisible particle of matter postulated by certain ancient philosophers as the fundamental constituent of matter See also atomism
a very small amount or quantity; minute fragment: to smash something to atoms, there is not an atom of truth in his allegations
Word Origin
C16: via Old French and Latin, from Greek atomos (n), from atomos (adj) that cannot be divided, from a-1 + temnein to cut
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
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Word Origin and History for atom

late 15c., as a hypothetical indivisible body, the building block of the universe, from Latin atomus (especially in Lucretius) "indivisible particle," from Greek atomos "uncut, unhewn; indivisible," from a- "not" + tomos "a cutting," from temnein "to cut" (see tome). An ancient term of philosophical speculation (in Leucippus, Democritus), revived 1805 by British chemist John Dalton. In late classical and medieval use also a unit of time, 22,560 to the hour. Atom bomb is from 1945 as both a noun and a verb; cf. atomic.

Online Etymology Dictionary, © 2010 Douglas Harper
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atom in Medicine

atom at·om (āt'əm)

  1. A unit of matter, the smallest unit of an element, having all the characteristics of that element and consisting of a dense, central, positively charged nucleus surrounded by a system of electrons. The entire structure has an approximate diameter of 10-8 centimeter and characteristically remains undivided in chemical reactions except for limited removal, transfer, or exchange of certain electrons.

  2. This unit regarded as a source of nuclear energy.

  3. A part or particle considered to be an irreducible constituent of a specified system.

  4. The irreducible, indestructible material unit postulated by ancient atomism.

  5. An extremely small part, quantity, or amount.

a·tom'ic (ə-tŏm'ĭk) adj.
The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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atom in Science
The smallest unit of an element, consisting of at least one proton and (for all elements except hydrogen) one or more neutrons in a dense central nucleus, surrounded by one or more shells of electrons. In electrically neutral atoms, the number of protons equals the number of electrons. Atoms remain intact in chemical reactions except for the removal, transfer, or exchange of certain electrons. Compare compound. See also ion, isotope, orbital.

The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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atom in Culture

atom definition

A unit of matter; the smallest unit of a chemical element. Each atom consists of a nucleus, which has a positive charge, and a set of electrons that move around the nucleus. (See Bohr atom.)

Note: Atoms link together to form molecules.
The New Dictionary of Cultural Literacy, Third Edition
Copyright © 2005 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
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