The Collaborative International Dictionary of English v.0.48:

Tau \Tau\ (tou), n. [Gr. tay^.]
   The nineteenth letter ([Tau], [tau]) of the Greek alphabet,
   equivalent to English t.
   [Webster 1913 Suppl.]

The Collaborative International Dictionary of English v.0.48:

Tau \Tau\, n. [Gr. tay^ the letter [tau] (English T).] (Zool.)
   The common American toadfish; -- so called from a marking
   resembling the Greek letter tau ([tau]).
   [1913 Webster]

   Tau cross. See Illust. 6, of Cross.
      [1913 Webster]