down quark

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The flavor of quark that has a charge of − ¹/₃ and a mass about 10 times that of an electron, belongs to the first generation of elementary fermions, and is a component of nucleons and other hadrons.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A quark having a fractional electric charge of -1/3 and a mass about 4 to 8 MeV. Symbol: d

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a stable quark with an electric charge of -1/3 and a mass 607 times that of an electron