Alternative Minimum Tax - AMT: Definition from Answers.com

Financial & Investment Dictionary: Alternative Minimum Tax (AMT)
Federal tax aimed at ensuring that wealthy individuals, trusts, estates and corporations pay at least some income tax. For individuals, the AMT is computed by adding Tax Preference Items to taxable income and making various adjustments to a taxpayer's regular taxable income. Taxpayers must calculate their tax obligations through the regular tax system and through the AMT system, and pay the greater of the two amounts.

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Taxpayers may have to pay the alternative minimum tax if their taxable income for regular tax purposes, combined with these adjustments and tax preference items, is more than $58,000 for married couples filing jointly, $40,250 for those filing as single or head of household, or $29,000 if filing singly and married on a separate return. The AMT tax rate on income up to $175,000 ($87,500 for a married couple filing separately) is 26% and 28% for income over $175,000.
Calculating the AMT can be extremely complex and is best left to a professional accountant.

Alternative Minimum Tax - AMT: Definition from Answers.com