atom

noun

at·​om ˈa-təm 

1

a

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

an atom of hydrogen

b

: 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

: a tiny particle : bit

There's not an atom of truth in what he said.

3

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

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.

Synonyms of atom

• particle
• speck
• molecule
• fleck
• bit
• grain
• patch
• snippet
• scrap

Examples of atom in a Sentence

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

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

Recent Examples on the Web

This has led scientists to suggest a whole host of hypothetical particles to account for dark matter, which go beyond the standard model of particle physics and the electrons, protons and neutrons that make up the atoms that compose all everyday matter, like stars, planets, moons and our bodies. —Robert Lea, Space.com, 9 Jan. 2026

These oxidants then break down the metal atoms, and allow the chloride ions to bind to them and dissolve them into the solution for easy recovery. —Georgina Jedikovska, Interesting Engineering, 8 Jan. 2026

Hydrogen atoms in groundwater, which filled cracks and pores in the surrounding rock, slowed the fast-moving neutrons released by fission just enough to trigger further atomic splits and keep the reaction going. —New Atlas, 8 Jan. 2026

In saturated fats, which are commonly found in meat, cheese and butter, the carbon molecules in the chain are linked with single carbon bonds and contain so many hydrogen atoms that the chain lies flat. —Lauren J. Young, Scientific American, 7 Jan. 2026

Word History

Etymology

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

First Known Use

15th century, in the meaning defined at sense 3

Phrases Containing atom

• atom smasher

Rhymes for atom

• datum
• stratum
• erratum
• striatum
• substratum
• heteroatom
• petrolatum

Kids Definition

atom

noun

at·​om ˈat-əm 

1

: a tiny particle : bit

2

: the smallest particle of an element that has the properties of the element and can exist either alone or in combination

3

: the atom considered as a source of vast potential energy

Etymology

Middle English atom "particle," from Latin atomus (same meaning), derived from Greek atomos "unable to be divided," from a- "not" and temnein "to cut"

Medical Definition

atom

noun

at·​om ˈat-əm 

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

atomic

ə-ˈtäm-ik 

adjective

atomically

-i-k(ə-)lē 

adverb