A meson is a type of hadron composed of one quark and one antiquark. Equivalently, a meson is a particle that is both a hadron
and a boson, or any particle whose decay products
do not include a baryon.
The commonest types of mesons are pions (pi mesons) and kaons (K mesons). These are also the only types of mesons which are long-lived enough to be seen directly by their tracks in a detector. More massive mesons with the same quark content but higher angular momentum (such the rho meson), as well as others containing one or more of the more massive quark types (such as the eta meson), are all very short lived. They have been found by studies of their decay products, but they decay too quickly to leave a track that can be detected.
Mesons can be positively or negatively charged, or electrically neutral. All are unstable with half-lives of ~10-8 second to ~10-23 second, and decay into stable particles. They are a major component of secondary cosmic rays.
Originally, a meson was considered to be any particle whose mass was between that of an electron and a proton. The name "meson" was coined in 1939 by the Indian physicist H. J. Bhaba. Ironically, so-called mu-mesons, or muons, which were discovered before pions, kaons, or eta-mesons, are not mesons according to the modern definition but are actually leptons.