Definition of atom
1a
: 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
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Merriam-Webster unabridged
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Time Traveler for atom
The first known use of atom was in the 15th century
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