Atom
Etymology
From Greek "atomos" — "a" (not) plus "temnein" (to cut) — meaning “uncuttable,” the smallest possible piece imagined by Democritus. The name stuck even after physicists split the atom, revealing protons, neutrons, and electrons inside.
noun · /AT-um/
The smallest unit of a chemical element that still counts as that element.
What it names
The basic building block of ordinary matter, the piece that chemistry rearranges but does not create or destroy.
Example
A single grain of sand contains more atoms than there are grains of sand on all the world’s beaches.
Related idea
An atom is a single element’s unit; a molecule is two or more atoms bonded together. One oxygen atom is not the oxygen you breathe — that is a molecule of two atoms joined.