quark

quark Any one of six particles (although there is increasing evidence of another two) and their antiparticles (antiquarks) that are the constituents of the hadron group of elementary particles. They occur in one of six ‘flavours’: up, down, top, bottom, charmed, and strange. Antiquarks have similar flavours but their charge is opposite that of their corresponding quark. Quarks always exist in combination; free quarks cannot exist. A baryon, such as a proton or neutron, consists of three.

quark A fundamental particle, of which hadrons are believed to be composed. Quarks have charges that are either +⅔ or −⅓ of the electron's charge, and combine to make elementary particles. Baryons consist of three quarks, and mesons of two. For example, a proton consists of quarks with charges +⅔+⅔−⅓= 1 and a neutron consists of three with charges +⅔−⅔−⅓ = 0. Eighteen different quarks with various properties are thought to exist, with a corresponding number of antiquarks. An isolated quark has never been observed; quarks appear to exist only in combination as baryons or mesons.