Atom
Definition
The smallest unit of an element that still keeps that element's chemical identity. Everything you touch is built from atoms.
Technical definition
An atom is the smallest electrically neutral unit of a chemical element, consisting of a dense nucleus of protons and neutrons surrounded by electrons in quantized energy levels. Chemical reactions rearrange atoms without changing their nuclear identity.
Example
A single gold atom is the smallest piece of gold that is still gold. Break it apart further and you get subatomic particles, not gold.
Common confusion
Atoms are not indivisible, they contain protons, neutrons, and electrons. They are also vastly smaller than cells: millions of atoms fit across the width of a human hair.