What is an 'Income Tax' An income tax is a tax that governments impose on financial income generated by all entities within their jurisdiction. By law, businesses and individuals must file an income tax return every year to determine whether they owe any taxes or are eligible for a tax refund. A tax that governments impose on financial income generated by all entities within their jurisdiction. By law, businesses and individuals must file an income tax return every year to determine whether they owe any taxes or are eligible for a tax refund. Income tax is a key source of funds that the government uses to fund its activities and serve the public.