Quark

A type of fundamental particle that forms the constituents of hadrons. There are currently believed to be six types (or flavours) of quarks (and their antiquarks): up, down, charm, strange, top, bottom. In quark theory, the baryon is composed of three quarks of different colour, an antibaryon is composed of three antiquarks, and a meson is composed of a quark and an antiquark. No quark has been observed in isolation.