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Any of a family of subatomic particles that are composed of a quark and an antiquark. Their masses are generally intermediate between leptons and baryons, and they can have positive, negative, or neutral charge. Mesons form a subclass of hadrons and include the kaon, pion and J/psi particles. Mesons were originally believed to be the particles that mediated the strong nuclear force, but it has since been shown that the gluon mediates this force. See Table at subatomic particle.