Particle Physics a type of quark with a mass of c. 0.005 to 0.015 GeV/c, a negative charge that is the charge of an electron, zero charm, and zero strangeness
See down quark in American Heritage Dictionary 4
noun Abbr. d
A quark with a charge of - 1/3 , a mass about 20 times that of the electron, and a downward spin. It is a component of protons and neutrons. See Table at subatomic particle.