:the smallest particle of an element that can exist either alone or in combination
: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
:a tiny particle :bit
- There's not an atom of truth in what he said.
:one of the minute indivisible particles of which according to ancient materialism (see materialism 1a) the universe is composed