A tax added to the normal tax paid by corporations or individuals who have earned income above a certain level. Bloomberg Financial Dictionary
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an additional tax paid by people or companies with a high level of income:
surtax on
• There are plans to impose a 10% surtax on incomes of more than $100,000.
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surtax UK US /ˈsɜːtæks/ US /ˈsɝː-/ noun [U]
► TAX an extra tax on income above a particular high level, or an extra tax on something that is already taxed: »
In addition, a 10% surtax applies to certain high-income taxpayers.
»He'd prefer to see an increase to the surtax rather than the base tax rate.
Financial and business terms. 2012.