Greek letter T (3)

I believe the answer is:
tau
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">greek letter t</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for tau that I've seen before include "19th letter of the Greek alphabet" , "Greek letter; cross" , "Marathon character" , "Letter from Greece" , "character from Athens" .)