Yes, tax is a Scrabble word!
...and is worth 10 points in Scrabble, and 10 points in Words with Friends
tax
Verb
1. Levy a tax on
2. Set or determine the amount of (a payment such as a fine)
3. Use to the limit
4. Make a charge against or accuse
Noun
1. Charge against a citizen's person or property or activity for the support of government
What Scrabble words can I make with the letters in tax?
2 Letter Words