A meson is a composite particle comprised of two quarks,
generally a quark and an antiquark (a quark's antimatter-equivalent).
There are numerous kinds of mesons, depending upon the combination
of quarks, that very in mass and electric charge, one type being
the pion.
Note that at some point in its history, particle physics
refined the definition of a meson as more details of
particle-interaction was discovered, such that some particles
previously classified as mesons no longer fit the definition.
For example, before this change, the muon was considered a type
of meson.