from The American Heritage® Dictionary of the English Language, 4th Edition

  • n. 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License

  • n. A quark having a fractional electric charge of +2/3 and a mass of about 1.5 to 4 MeV. Symbol: u

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

  • n. a stable quark with an electric charge of +2/3 and a mass 607 times that of an electron


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