A lump-sum tax is a tax that is a fixed amount no matter what the change in circumstance of the taxed entity. (A lump-sum subsidy or lump-sum redistribution is defined similarly.)
It is one of the various modes used for taxation: income, things owned (property taxes), money spent (sales taxes), miscellaneous (excise taxes).
It is a regressive tax, such that the lower income is, the higher percentage of income applicable to the tax. An example is a poll tax to vote, which is unchanged no matter what the income of the voter.
In economic theory, a lump-sum tax may have the advantage of not contributing to an excess burden of taxation, a loss in economic efficiency that results from taxes reducing incentives for production. In practice, lump-sum taxes are often encountered, in spite of their conflict with other criteria, such as equity or ability to pay. A lump-sum tax remains a standard for measuring the performance of other imperfect kinds of taxes (J. de V. Graaf, 1987).