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 cross
    n 1: cross resembling the Greek letter tau [syn: tau cross,
         St. Anthony's cross]