A tax added to an existing tax. To help finance the Vietnam War, for example, Congress imposed a surtax on the federal income tax.
Surtax
An additional tax on income paid by an individual or corporation.
Investopedia Commentary
For example, during the Vietnam War U.S. Congress levied a surtax on income to finance the war effort. This added to the animosity towards the war.
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See also: Corporate Tax, Direct Tax, Income Tax, Indirect Tax, Self-Employment Tax