Hadron, name for two of the four basic classes of subatomic particles. These four classes, from lightest to heaviest, are: bosons, leptons, mesons, and baryons. Mesons and baryons are considered hadrons. Mesons are the particles that hold atomic nuclei together. Baryons are the largest class of particles; both protons and neutrons, which are the main particles of nuclei, are baryons. The reason that mesons and baryons are sometimes considered together as hadrons is that both are subject to the strong nuclear force (one of the four basic forces of nature; the others being gravitations, electromagnetisms, and the weak nuclear force). The other two classes of particles—bosons and leptons—are not subject to the strong nuclear force. Current theory assumes that all hadrons are composed of more basic particles, called quarks.