Particle Physics a type of quark with a mass of c. 0.002 to 0.008 GeV/c, a positive charge that is that of an electron, zero charm, and zero strangeness
See up quark in American Heritage Dictionary 4
noun Abbr. u
A quark with a charge of + 2/3 and a mass about 607 times that of the electron. It is a component of protons and neutrons. See Table at subatomic particle.