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]

WordNet (r) 3.0 (2006):

tau
    n 1: the 19th letter of the Greek alphabet

The Free On-line Dictionary of Computing (30 December 2018):

tau

    The mathematical constant that is the circumference
   of a circle divided by its radius, equal to twice pi.

   Some have suggested that pi is the wrong choice of constant to
   describe the geometry of circles etc. and tau = 2*pi would have
   been better.  Most practising mathematicians think this is silly.
   The xkcd comic strip facetiously proposed
   (http://xkcd.com/1292/) a compromise of 1.5*pi or "pau".

   The Pi Manifesto (http://www.thepimanifesto.com/).

   (2013-12-10)