Any "compulsory contribution to state revenue, levied by the government on workers' income and business profits or added to the cost of some goods, services, and transactions" is a tax.
Is the penalty in Obamacare a "compulsory contribution to state revenue, levied by the government on workers' income and business profits or added to the cost of some goods, services, and transactions"? Yes. So the penalty is a tax because it fits the definition of a tax, even if it isn't called a tax.