Any of a class of subatomic particles that interact by the strong interaction (see fundamental interactions). The class includes protons, neutrons, and pions. Hadrons are believed to have an internal structure and to consist of quarks; they are therefore not truly elementary. Hadrons are either baryons, including protons, and are believed to consist of three quarks, or mesons, which decay into leptons and photons and are believed to consist of a quark and an antiquark. See elementary particles.