Definition of tax as a compulsory, enforceable public exaction clarifies which fiscal impositions qualify under income-tax law. The term tax for income-tax law includes income-tax and, in specified assessment years, other statutory fiscal charges. A tax is defined by five characteristics: compulsory exaction by a public authority; imposition under statutory power without consent; enforceability by law; levy for public purposes rather than special benefit to the payer; and being part of the common burden with incidence tied to capacity to pay.