A term in physics for a fundamental constituent of matter is a particle. In modern physics, a particle is an individual unit used to describe matter or radiation, although quantum theory shows that particles can also behave like waves.
Some particles are elementary, meaning they are not known to be made of smaller components. Examples in the Standard Model include electrons, neutrinos, quarks, photons, gluons, and the Higgs boson. Other particles are composite: protons and neutrons, for example, are made from quarks bound together by gluons.
The word “particle” does not always mean a tiny solid ball. Quantum particles are better understood as excitations or localized measurements of underlying fields. Their properties can include mass, electric charge, energy, spin, and lifetime.
A common mix-up is treating atoms as the smallest possible particles. Atoms contain electrons and a nucleus, while the nucleus contains protons and neutrons. Those discoveries transformed the older idea of the atom as indivisible and led to modern particle physics.