Not to be confused with a hard-on, the hadron is the most common type of subatomic particle. A particle is considered a hadron when it is held together and interacts mainly via the strong nuclear force. Hadrons are divided into two groups: the mesons and the baryons. Various baryons include the proton, neutron, positive sigma, neutral sigma, and neutral lambda. Mesons include the positive, negative, and neutral pions and kaons, and the positive and negative D mesons.
All hadrons apart from the proton (and the neutron when it is bound in a stable nuclide) are known to be unstable: they can decay into other hadrons (hadronic decay modes), into leptons (leptonic decays), into both (semileptonic decays) or can decay via emission of photons (electromagnetic decays).