Baryon particle. Computer illustration showing the quantum structure of a baryon particle. A baryon is a composite subatomic particle and one of two forms of hadron, the other being the meson. Hadrons are composed of quarks (round), held together by the strong force (coils). Baryons are composed of three quarks, whilst mesons consist of two quarks. The protons and neutrons found in all atomic nuclei are both baryons.