Alternative Minimum Tax. An IRS mechanism created to ensure that high-income individuals, corporations, trusts, and estates pay at least some minimum amount of tax, regardless of deductions, credits or exemptions. It operates by adding certain tax-preference items back into adjusted gross income. While it was once only important for a small number of high-income individuals who made extensive use of tax shelters and deductions, more and more people are being affected by it. The AMT is triggered when there are large numbers of personal exemptions on state and local taxes paid, large numbers of miscellaneous itemized deductions or medical expenses, or by Incentive Stock Option (ISO) plans.