A type of particle. Mesons and baryons are the two types of hadron. All hadrons are (experimental data seems to confirm) made up of quarks, as independently theorized by Murray Gell-Mann and George Zweig in 1964. Mesons are made up of one quark and one antiquark. Some types of meson are pion, kaon, rho, B-zero and eta-c. There are 140 types of meson.