noun at·om \ˈa-təm\

Definition of atom

  1. 1a :  the smallest particle of an element that can exist either alone or in combination an atom of hydrogenb :  the atom considered as a source of vast potential constructive or destructive energy … a largely forgotten legacy of this country's conquest of the atom. — William J. Broad … when Congress passed the Atomic Energy Act in 1954 and allowed private utilities to “harness the atom.” — Barry Werth

  2. 2 :  a tiny particle :  bit There's not an atom of truth in what he said.

  3. 3 :  one of the minute indivisible particles of which according to ancient materialism (see materialism 1a) the universe is composed

Examples of atom in a sentence

  1. There is not an atom of truth to what he said.

  2. give me just one atom of information about the novel's surprise ending

Did You Know?

Some ancient philosophers believed that matter is infinitely divisible, that any particle, no matter how small, can always be divided into smaller particles. Others believed that there must be a limit and that everything in the universe must be made up of tiny indivisible particles. Such a hypothetical particle was called atomos in Greek, which means “indivisible.” According to modern atomic theory, all matter is made up of tiny particles named atoms from the ancient Greek atomos. However, it has turned out that atoms are not indivisible after all. Indeed, the splitting of atoms can be used to produce vast amounts of energy, as in atom bombs.

Origin and Etymology of atom

Middle English, from Latin atomus, from Greek atomos, from atomos indivisible, from a- + temnein to cut

First Known Use: 15th century

ATOM Defined for English Language Learners


noun at·om \ˈa-təm\

Definition of atom for English Language Learners

  • : the smallest particle of a substance that can exist by itself or be combined with other atoms to form a molecule

  • : a very small amount of something

ATOM Defined for Kids


noun at·om \ˈa-təm\

Definition of atom for Students

  1. 1 :  the smallest particle of an element that can exist alone or in combination carbon atoms

  2. 2 :  a tiny particle :  bit “I don't believe there's an atom of meaning in it.” — Lewis Carroll, Alice's Adventures in Wonderland

History for atom

The English word atom came from a Greek word atomos, meaning “not able to be divided.” Ancient Greek philosophers believed that matter could only be divided to the very smallest particles, atoms, which could not be further divided. Modern science revived the atom idea, but it was discovered that even atoms could be split, and that doing so produced great amounts of energy.

Medical Dictionary


noun at·om \ˈat-əm\

Medical Definition of atom

  1. :  the smallest particle of an element that can exist either alone or in combination


\ə-ˈtäm-ik\play adjective


\-i-k(ə-)lē\play adverb

Seen and Heard

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a trip made at another's expense

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