Solve the Unsolved: Intriguing Letter Between Sigma & Upsilon!

– Greek letter after sigma
– Greek letter before upsilon
– It comes after sigma and before upsilon
– 19th letter in the Greek alphabet
– Rhymes with “wow”
– Symbol used in mathematics and physics
– Commonly used in fraternity and sorority names
– Looks like a sideways “T”
– Pronounced like the word “tau”