The deep thinkers who are running the Romney campaign are arguing what you should call the provision of a law that prescribes a stick to encourage people to buy health insurance: a tax or a penalty. The Supreme Court seems to think it is an important distinction.
Suppose the Constitution prevents Congress from mandating behavior X. Then, according to our Ivy League masters, it can't impose a penalty on those individuals or states who fail to engage in behavior X. But somehow the taxing authority of Congress extends to taxing individuals, but not states, who fail to engage in behavior X. This may be interesting to the sophists who teach in our great law schools, but ordinary citizens should not be fooled. No matter what you call the tax/penalty, nobody will pay a dime more or less. The distinction is valuable to those who view constitutional provisions as mere obstacles to be gotten around by tricky language games, but not to those who end up paying the fee.
It is worthwhile to debate whether either the Commerce Clause or the various taxing powers of Congress can be used to compel or discourage behavior that Congress has no constitutional power to simply mandate. However, once the tricksters have gotten around the obstacle, what difference does it make what the rest of us call it?
Is it a penalty or a tax? The left is rolling in the aisles laughing. Another Romney flip-flop, they scold. This would be a good opportunity to appeal to the good sense of voters and tell them that you can call a flower a rose or a stinkweed, but it will still smell the same. (Apologies to the Bard.)