The term 'quark' is a PARTICLE that is one of the fundamental building blocks of matter. It was first proposed by physicist Murray Gell-Mann in 1964 and is part of the Standard Model of PARTICLE physics. Quarks come in six different 'FLAVORS': up, down, strange, charm, bottom, and top. They are held together by the strong nuclear force and interact with each other through the exchange of gluons. Quarks are never found in isolation, but instead form composite particles such as protons and neutrons.