the nineteenth letter of the Greek alphabet (Τ, τ)
See tau in American Heritage Dictionary 4
(tou, tô)
noun
The 19th letter of the Greek alphabet. See Table at alphabet.
An elementary particle of the lepton family, having a mass about 3,550 times that of the electron, a negative electric charge, and a mean lifetime of 3 × 10-13 seconds. See Table at subatomic particle.