To tax (from the Latin taxo; "I estimate") is to impose a financial charge or other levy upon a taxpayer (an individual or legal entity) by a state or the functional equivalent of a state such that failure to pay is punishable by law. Taxes are also imposed by many administrative divisions. Taxes consist of direct tax or indirect tax, and may be paid in money or as its labour equivalent (often but not always unpaid labour).

A tax may be defined as a "pecuniary burden laid upon individuals or property owners 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."

The legal definition and the economic definition of taxes differ in that economists do not consider many transfers to governments to be taxes. For example, some transfers to the public sector are comparable to prices. Examples include tuition at public universities and fees for utilities provided by local governments. Governments also obtain resources by creating money (e.g., printing bills and minting coins), through voluntary gifts (e.g., contributions to public universities and museums), by imposing penalties (e.g., traffic fines), by borrowing, and by confiscating wealth. From the view of economists, a tax is a non-penal, yet compulsory transfer of resources from the private to the Public sector levied on a basis of predetermined criteria and without reference to specific benefit received.