A proportional tax refers to a tax system where the tax rate remains the same regardless of the income level of the taxpayer. In other words, everyone pays the same percentage of their income as tax. For example, if the tax rate is 10%, then someone who earns $10,000 will pay $1,000 in taxes while someone who earns $100,000 will pay $10,000 in taxes. Therefore, the tax rate is proportional to the income level.