molecule
A molecule is what you get when two or more atoms link arms and hold on — a tiny, settled group that behaves as one. Think of atoms as letters and molecules as words: on their own the letters mean little, but joined in the right order they spell something real. Two hydrogen atoms clasped to one oxygen atom spell H₂O — a single drop of water's worth of meaning, repeated countless times over.
A molecule is also the smallest piece of a substance that still acts like that substance. Split a water molecule and you no longer have water at all — just hydrogen and oxygen, two gases with utterly different habits. The wetness, the way it freezes and boils, the way it dissolves sugar: all of that lives in the whole molecule, not in its separate atoms.
A common mix-up: molecules aren't always made of different elements. Two identical atoms count too — the oxygen you breathe travels as O₂, a molecule of two oxygen atoms holding hands. "Molecule" just means atoms bonded together; whether they're the same or different is a separate question.
Water: two hydrogen atoms bonded to one oxygen atom — one molecule.
The word comes from the Latin molecula, "a little mass" — a tiny diminutive of moles, "mass." It entered science in the 1600s, but the modern idea that matter is built from atoms joined into molecules was only nailed down in the 1800s.