Rob Natelson explains it all in his latest blog post. Short answer: if the purpose of the tax is raising revenue (e.g., the Stamp Act), it’s a tax. If the purpose is the regulation of commerce (e.g., a prohibitive tariff on imported French clothing; a shipping tax dedicated to paying for harbor improvements), then it’s not a “tax” in the the constitutional sense. Rather, it is a regulation of commerce.
The American colonists believed that Parliament had full authority to regulate external commerce, such as by imposing protectionist tariffs. The colonists also believed that Parliament had no authority to impose domestic taxes in the colonies (such as the Stamp Act). The colonists had a very firm sense of the distinction, and ended up going to war over Parliament’s refusal to respect that distinction. Because the Obamacare mandate is designed purely to control behavior, and not to raise revenue (even if it, like a protectionist tariff on French clothing does ultimately raise a little revenue), the Obamacare mandate is a type of commerce regulation, and not a tax in the constitutional sense. That, at least, is what the original meaning tells us.
Of course whether the individual mandate actually qualifies as a regulation of “commerce...among the several States” is a separate issue. The original meaning question for the mandate’s penalty is a commerce issue, not a tax issue.