Yes, tau is a Scrabble word!
...and is worth 3 points in Scrabble, and 4 points in Words with Friends
tau
Noun
1. The 19th letter of the Greek alphabet
What Scrabble words can I make with the letters in tau?
3 Letter Words
2 Letter Words
Noun
1. The 19th letter of the Greek alphabet
3 Letter Words
2 Letter Words