Any of the six different varieties of quarks. All hadronic matter is composed of quarks, the most elementary constituents of matter. The six different flavours are labelled u, d, s, c, b, and t, corresponding to up, down, strange, charmed, bottom, and top. Quarks are all spin-½ fermions. The u, c, and t flavours carry a positive electric charge equal in magnitude to two-thirds that of the electron; the d, s, and b flavours have a negative charge one-third that of the electron.

Different flavoured quarks have vastly different masses ranging from the lightest, the u quark, with a mass around 5 MeV/c² (where c is the speed of light), equal to the mass of about 10 electrons, to the top quark, with a mass 35,000 times greater, or 175 GeV/c², about the mass of a gold atom. Because of its large mass, only in 1994 was evidence presented for existence of the top quark. Quarks of any flavour are further characterised by three additional quantum numbers called color: red, green, and blue. Each quark has an antiquark counterpart carrying the corresponding anticolor. All of these properties have been well established by experiment.