A method of figuring income tax by paying tax on the average income per year for the past five years. For example: A, a real estate salesperson, earns $10,000 taxable income for 4 years. In the fifth year, A sells a shopping center and earns $100,000 taxable income. A could take the total income for 5 years ($140,000), divide by 5 ($28,000), and pay tax on $28,000 for the past 5 years, less what A has already paid.