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Definition of QUARK
: any of several elementary particles that are postulated to come in pairs (as in the up and down varieties) of similar mass with one member having a charge of +2⁄3 and the other a charge of −1⁄3 and are held to make up hadrons
Any of a group of subatomic particles thought to be among the fundamental constituents of mattermore specifically, of protons and neutrons. The concept of the quark was first proposed by Murray Gell-Mann and George Zweig (b. 1937); its name was taken from James Joyce's novel Finnegans Wake. Quarks include all particles that interact by means of the strong force. They have mass and spin, and they obey the Pauli exclusion principle. They have never been resolved into smaller components, and they never occur alone. Their behaviour is explained by the theory of quantum chromodynamics, which provides a means of calculating their basic properties. There are six types of quarks, called up, down, strange, charm, bottom, and top. Only the up and down quarks are needed to make protons and neutrons; the others occur in heavier, unstable particles.