A tax is a financial charge or other levy imposed on an individual or a legal entity by a state or a functional equivalent of a state (for example, secessionist movements or revolutionary movements). Taxes are also imposed by many subnational entities. Taxes consist of direct tax or indirect tax, and may be paid in money or as its labour equivalent (often but not always unpaid). A tax may be defined as a "pecuniary burden laid upon individuals or property to support the government […] a payment exacted by legislative authority." A tax "is not a voluntary payment or donation, but an enforced contribution, exacted pursuant to legislative authority" and is "any contribution imposed by government […] whether under the name of toll, tribute, tallage, gabel, impost, duty, custom, excise, subsidy, aid, supply, or other name."

In modern taxation systems, taxes are levied in money, but in-kind and corvée taxation are characteristic of traditional or pre-capitalist states and their functional equivalents. The method of taxation and the government expenditure of taxes raised is often highly debated in politics and economics. Tax collection is performed by a government agency such as Canada Revenue Agency, the Internal Revenue Service (IRS) in the United States, or Her Majesty's Revenue and Customs (HMRC) in the UK. When taxes are not fully paid, civil penalties (such as fines or forfeiture) or criminal penalties (such as incarceration) may be imposed on the non-paying entity or individual.

Economics of taxation

In economic terms, taxation transfers wealth from households or businesses to the government of a nation. The side-effects of taxation and theories about how best to tax are an important subject in microeconomics. Taxation is almost never a simple transfer of wealth. Economic theories of taxation approach the question of how to minimize the loss of economic welfare through taxation and also discuss how a nation can perform redistribution of wealth in the most efficient manner.

Deadweight costs of taxation

For goods supplied in a perfectly competitive market, tax reduces economic efficiency, by introducing a deadweight loss. In a perfect market, the price of a particular economic good adjusts to make sure that all trades which benefit both the buyer and the seller of a good occur. After introducing a tax, the price received by the seller is less than the cost to the buyer. This means that fewer trades occur and that the individuals or businesses involved gain less from participating in the market. This destroys value, and is known as the 'deadweight cost of taxation'.

The deadweight cost is dependent on the elasticity of supply and demand for a good.

Most taxes—including income tax and sales tax—can have significant deadweight costs. The only way to completely avoid deadweight costs in an economy which is generally competitive is to find taxes which do not change economic incentives, such as the land value tax, where the tax is on a good in completely inelastic supply, a lump sum tax such as a poll tax which is paid by all adults regardless of their choices, or a windfall profits tax which is entirely unanticipated and so cannot affect decisions.

Double dividend taxes

In some cases where the economy is not perfectly competitive, the existence of a tax can increase economic efficiency. If there is a negative externality associated with a good, meaning that it has negative effects not felt by the consumer, then the free market will trade too much of that good. By putting a tax on the good, the government can increase overall welfare as well as raising revenue in taxation. This is known as a 'double dividend'.

There are a wide range of goods where there is, or is claimed to be, a negative externality. Polluting fuels (like petrol), goods which incur public healthcare costs (such as alcohol or tobacco), and charges for existing 'free' public goods (like congestion charging) all offer the possibility of a double dividend. This type of tax is a Pigovian tax, sometimes colloquially known as a 'sin tax'. It is worthwhile noting that taxation is not necessarily the only, or the best, method of dealing with negative externalities.

Transparency and simplicity

Another concern is that the complicated tax codes of developed economies offer perverse economic incentives. The more details of tax policy there are, the more opportunities for legal tax avoidance and illegal tax evasion; these not only result in lost revenue, but involve additional deadweight costs: for instance, payments made for tax advice are essentially deadweight costs because they add no wealth to the economy. Perverse incentives also occur because of non-taxable 'hidden' transactions; for instance, a sale from one company to another might be liable for sales tax, but if the same goods were shipped from one branch of a corporation to another, no tax would be payable.

To address these issues, economists often suggest simple and transparent tax structures which avoid providing loopholes. Sales tax, for instance, can be replaced with a value added tax which disregards intermediate transactions.

Tax incidence

Economic theory suggests that the economic effect of tax does not necessarily fall at the point where it is legally levied. For instance, a tax on employment paid by employers will impact on the employee, at least in the long run. The greatest share of the tax burden tends to fall on the most inelastic factor involved - the part of the transaction which is affected least by a change in price. So, for instance, a tax on wages in a town will (at least in the long run) affect property-owners in that area.

Costs of compliance

Although governments must spend money on tax collection activities, some of the costs, particularly for keeping records and filling out forms, are borne by businesses and by private individuals. These are collectively called costs of compliance. More complex tax systems tend to have higher costs of compliance. This fact can be used as the basis for practical or moral arguments in favor of tax simplification (see, for example, FairTax), or tax elimination (in addition to moral arguments described above).