Final answer:
When the tax percentage of income remains constant with an increase in income, it is called a proportional tax, characterized by a flat tax rate irrespective of income.
Step-by-step explanation:
If with the increase in income, the percentage of income collected as tax remains constant, the tax will be called a proportional tax. A proportional tax is defined as a tax that is a flat percentage of income earned, regardless of the level of income. This is in contrast to a regressive tax, where higher income earners pay a smaller share of their income in tax, and to a progressive tax, where the tax rate increases as the income of the taxpayer increases.