As I recall, they carefully left out any mention of a tax in the ACA. But you’re forced to buy health insurance or pay a penalty if you don’t & is enforced through the IRS. You can’t opt into Obamacare without revealing your income, because any subsidy is going to be based on what is on your tax return.
Also, recall that Johnny Roberts had to decide the mandate was a tax in order to convince himself to let the ACA stand the first time around.
Moreover, the ACA contains plenty of literal taxes, such as the 2.3% tax on medical device sales, the surtax on investment income, the 40% "cadillac tax" on insurance plans deemed to be too good, the 10% tax on indoor tanning services, and a number of other direct and indirect taxes.
However the administration argued before the Supreme Court that it was a tax. The court ruled in their favor and it is now adjudicated before the highest court in the land that it is a tax. However, I fully expect John Roberts to screw us again. If he does this is grounds for impeachment in any rational court.