What is Alternative Minimum Tax?

Legal Definition
The alternative minimum tax (AMT) is a supplemental income tax imposed by the United States federal government required in addition to baseline income tax for certain individuals, corporations, estates, and trusts that have exemptions or special circumstances allowing for lower payments of standard income tax. AMT is imposed at a nearly flat rate on an adjusted amount of taxable income above a certain threshold (also known as exemption). This exemption is substantially higher than the exemption from regular income tax.

Regular taxable income is adjusted for certain items computed differently for AMT, such as depreciation and medical expenses. No deduction is allowed for state taxes or miscellaneous itemized deductions in computing AMT income. Taxpayers with incomes above the exemption whose regular Federal income tax is below the amount of AMT must pay the higher AMT amount.

A predecessor "minimum tax", enacted in 1969, imposed an additional tax on certain tax benefits for certain taxpayers. The present AMT was enacted in 1982 and limits tax benefits from a variety of deductions. On January 2, 2013, President Barack Obama signed the American Taxpayer Relief Act of 2012, which indexes to inflation the income thresholds for being subject to the tax.
-- Wikipedia
Legal Definition
A tax mechanism that calculates taxable income after certain deductions and tax preferences are made. The AMT ensures that high-income individuals and entities pay a certain amount of income despite favorable tax treatment.
Legal Definition
The way US taxes make sure rich parties pay a minimum despite breaks. They must file with and without breaks and pay the highest tax.