What is Proportional tax?
Also known as: single-rate (flat) tax
A tax under which the same rate applies regardless of the size of the tax base. If the base doubles, the amount of tax also exactly doubles, but its share of the base does not change. It is easy to calculate and monitor; however, because low- and high-income payers pay the same share of the base, it does not reduce income inequality. An example of a single rate is the general VAT rate: 12% regardless of the amount of the sale.
Source: Britannica: Proportional, progressive, and regressive taxes