1. the 19th letter of the Greek alphabet;

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 alphabetThe Free On-line Dictionary of Computing (18 March 2015):

tauThe 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)