In physics, a meson is a non-elementary particle (hadron) composed of a quark and an antiquark of opposite color charge. Often a given quark-antiquark pair does not occur on its own, but instead is mixed with others so that the quarks have a superposition of flavors (as always, flavors more similar in mass mix more). Pseudoscalar mesons (spin 0) have the lowest energy, where the quark and antiquark have opposite spin, and then the vector mesons (spin 1), where the quark and antiquark have parallel spin. Both come in higher energy versions where the spin is augmented by orbital angular momentum. Most of a meson's mass comes from binding energy, rather than the sum of the mass of its components. All mesons are unstable.
Mesons were originally predicted as carriers of the force that bind protons and neutrons together. When first discovered, the muon was identified with this family from its similar mass and was named "mu meson", however it did not show a strong attraction to nuclear matter and is actually a lepton. Later the pion was discovered, which is the actual force carrier and decays into the muon.