Source : Webster's Revised Unabridged Dictionary (1913)

Tau \Tau\ (tou), n. [Gr. tay^.]
   The nineteenth letter ([Tau], [tau]) of the Greek alphabet,
   equivalent to English t.

Tau \Tau\, n. [Gr. tay^ the letter [tau] (English {T}).]
   (Zo["o]l.)
   The common American toadfish; -- so called from a marking
   resembling the Greek letter tau ([tau]).

   {Tau cross}. See Illust. 6, of {Cross}.

Source : WordNet®

tau
     n : the 19th letter of the Greek alphabet