Interesting nugget from Justice Kagan’s majority opinion in CSX Transportation, Inc. v. Alabama Dept. of Revenue:

The 4–R Act does not define “tax”; nor does the statute otherwise place any matters within, or exclude any matters from, the term’s ambit. In these circumstances, we look to the word’s ordinary definition, Asgrow Seed Co. v. Winterboer, 513 U. S. 179, 187 (1995), and we note what taxpayers have long since discovered—that the meaning of “tax” is expansive. A State (or other governmental entity) seeking toraise revenue may choose among multiple forms of taxation on property, income, transactions, or activities.

So the taxing power is “expansive,” and it applies to things like owning property, earning income, engaging in transactions, or simply, activities. Not inactivity. Got it. Good thing to keep in mind.