Molecule
Etymology
From Latin "molecula", a diminutive of "moles" (a mass or bulk) — literally a “little mass.” The word arrived in the 18th century as chemists needed a name for the small combined units of matter.
noun · /MOL-eh-kyool/
Two or more atoms bonded together, forming the smallest unit of a chemical compound.
What it names
The level where substances get their identity: water is not its hydrogen and oxygen atoms but the specific molecule they make together.
Example
A water molecule is two hydrogen atoms locked to one oxygen atom — split them and you no longer have water.
Related idea
A molecule is built from atoms, the way a word is built from letters. The atoms of hydrogen and oxygen are flammable and reactive; the molecule they form, water, puts out fires.