top quark

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A hypothetical quark with a charge of + ²/₃ and a mass of 360,000 times that of the electron. See Table at subatomic particle.

Wiktionary

1. n. A quark having a fractional electric charge of +2/3 and a mass about 178,000 MeV. Symbol: t

WordNet 3.0

1. n. a hypothetical quark with a charge of +2/3 and a mass more than 100,000 times that of an electron