Which term is used to describe an increase in tax rates as income increases?

The term that describes an increase in tax rates as income increases is progressive tax. This system is designed to impose a higher tax rate on individuals or entities with higher incomes, meaning that as a person's income rises, they pay a larger percentage of that income in taxes. This approach aims to reduce income inequality by redistributing wealth more effectively; higher earners contribute a greater share toward public services and government functions.

This system contrasts with a regressive tax, where the tax rate decreases as the income increases, placing a heavier burden on low-income earners. A flat tax, on the other hand, applies the same tax rate to all taxpayers regardless of income level, while an equitable tax isn't a standard term used in taxation, but would generally refer to a fair distribution of tax burdens, which doesn’t specifically relate to how rates change with income.