It's nice to be clear, since "tax" is also taken to mean, in a wider sense, as any money that you're obligated or "strongly encouraged" to spend as a result of government action.
So the increase in health care costs because of uninsured ER (and other) patients has been termed a "hidden tax." In some cases it's called that because it's an odd cost of doing business, not one that directly tied to doing business and voluntary.
In other cases, it's because a certain class of care givers are required by law to provide the care. In a real sense it doesn't matter if the government collects the funds directly or not if it's requiring that they be a part of doing business. It's like having a business required to put in time in record keeping for the government--it's not exactly voluntary and it's a cost of doing business, but not one that's necessary, in the strictest sense, for doing business.
The details of the definition concern me less than knowing what the definition in play is, and that it's a constant throughout the argument or text.
So in the narrowest sense, the tax from the ACA isn't all that large. Can't find if it's the same, in terms of GDP, how it stacks up against the 1937 tax increase.