WordReference Random House Learner's Dictionary of American English © 2017
at•om /ˈætəm/USA pronunciation   n. [countable]
  1. Physics
    • Physicsthe smallest part of an element that still has the chemical properties of the element.
    • this part of an element as the source of nuclear energy.
  2. a very small quantity: There was not even an atom of truth in that remark.
See -tom-.
WordReference Random House Unabridged Dictionary of American English © 2017
at•om  (atəm),USA pronunciation n. 
  1. Physics
    • 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.
    • an atom with one of the electrons replaced by some other particle:muonic atom; kaonic atom.
  2. Energythis component as the source of nuclear energy.
  3. a hypothetical particle of matter so minute as to admit of no division.
  4. anything extremely small;
    a minute quantity.
  • Greek átomos, noun, nominal use of átomos undivided, equivalent. to a- a-6 + tomós divided, verbid of témnein to cut
  • Latin atomus
  • Middle English attomos, athomus 1350–1400
    • 4.See corresponding entry in Unabridged shred, speck, scintilla, iota, jot, whit.

Collins Concise English Dictionary © HarperCollins Publishers::

atom /ˈætəm/ n
  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
  3. any entity regarded as the indivisible building block of a theory
  4. the hypothetical indivisible particle of matter postulated by certain ancient philosophers as the fundamental constituent of matter
  5. a very small amount or quantity; minute fragment: to smash something to atoms, there is not an atom of truth in his allegations
Etymology: 16th Century: via Old French and Latin, from Greek atomos (n), from atomos (adj) that cannot be divided, from a-1 + temnein to cut

'atom' also found in these entries:

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