What is Income Tax?

Legal Definition
An income tax is a tax imposed on individuals or entities (taxpayers) that varies with the income or profits (taxable income) of the taxpayer. Details vary widely by jurisdiction. Many jurisdictions refer to income tax on business entities as companies tax or corporate tax. Partnerships generally are not taxed; rather, the partners are taxed on their share of partnership items. Tax may be imposed by both a country and subdivisions. Most jurisdictions exempt locally organized charitable organizations from tax.

Income tax generally is computed as the product of a tax rate times taxable income. The tax rate may increase as taxable income increases (referred to as graduated rates). Taxation rates may vary by type or characteristics of the taxpayer. Capital gains may be taxed at different rates than other income. Credits of various sorts may be allowed that reduce tax. Some jurisdictions impose the higher of an income tax or a tax on an alternative base or measure of income.

Taxable income of taxpayers resident in the jurisdiction is generally total income less income producing expenses and other deductions. Generally, only net gain from sale of property, including goods held for sale, is included in income. Income of a corporation's shareholders usually includes distributions of profits from the corporation. Deductions typically include all income producing or business expenses including an allowance for recovery of costs of business assets. Many jurisdictions allow notional deductions for individuals, and may allow deduction of some personal expenses. Most jurisdictions either do not tax income earned outside the jurisdiction or allow a credit for taxes paid to other jurisdictions on such income. Nonresidents are taxed only on certain types of income from sources within the jurisdictions, with few exceptions.

Most jurisdictions require self-assessment of the tax and require payers of some types of income to withhold tax from those payments. Advance payments of tax by taxpayers may be required. Taxpayers not timely paying tax owed are generally subject to significant penalties, which may include jail for individuals or revocation of an entity's legal existence.
-- Wikipedia
Legal Definition
Income Tax Law: An Overview

In 1913, the Sixteenth Amendment to the U.S. Constitution was ratified. It empowered Congress to tax "incomes, from whatever source derived, without apportionment among the several States, and without regard to any census or enumeration." The Internal Revenue Code is today embodied as Title 26 of the United States Code (26 U.S.C.) and is a lineal descendant of the income tax act passed in 1913, following ratification of the Sixteenth Amendment. While some states do not have an income tax (Nevada), all residents and all citizens of the United States are subject to the federal income tax. Not everyone, however, must file a return. The requirements for filing are found in 26 U.S.C. § 6011. As the largest contributor, its purpose is to generate revenue for the federal budget. In 1985 for example, the government collected over $450 billion in income tax from a total of $742 billion in total internal revenue receipts. The funds collected are essential for the shaping and preservation of a free market economy.

Some terms are essential in understanding income tax law. "Gross income" can be generaly defined as "all income from whatever source derived;" a more complete definition is found in 26 U.S.C. § 61. Other important definitions like "taxable income" and "adjusted gross income" can also be found in Chapter I of Title 26. These terms are not fixed nor should anyone be confident in understanding their true meaning after a cursory reading because their imputed definitions change with time. The Supreme Court, through case law, demonstrates the changing meaning of taxable income.

Individuals are not the only ones required to file income tax returns. Corporations do as well. While they are subject to may of the same rules as are individual taxpayers, they are also covered by an intricate body of rules addressed to the peculiar problems of corporations.
Legal Definition
A tax on the yearly profits arising from property, professions, trades, and offices. 2 Steph. Comm. 573. Levi v. Louisville, 97 Ky. 394, 30 S. W. 973, 28 In In A. 480; Parker v. Insurance Co.., 42 La. Ann. 428, 7 South. 599.
-- Black's Law Dictionary
Legal Definition
A tax on the product of property or from business pursuits. See 97 Ky. 394, 28 L. R. A. 480, 30 S. W. 973.
-- Ballentine's Law Dictionary