atom

UK: /ˈætəm/ US: /ˈætəm/ ,(at′əm)

WordReference Random House Learner's Dictionary of American English © 2025

at•om /ˈætəm/   n. [countable]

1. Physics
   • Physics the 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 © 2025

at•om  (at′əm), 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. Energy this 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

Synonyms

• 4. shred, speck, scintilla, iota, jot, whit.

Collins Concise English Dictionary © HarperCollins Publishers::

atom / ˈætəm/ noun

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
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