AMT stands for Alternative Minimum Tax.  

There are a variety of exclusions, deductions and credits available to corporations that allow them to reduce the amount of tax they are required to pay on the income they earn.

AMT ensures that these corporations pay at least some tax on their income.  AMT rules disallow certain deductions that make it almost impossible for  anyone to avoid being taxed.

Once the deductions are eliminated, the tax is recalculated. If the AMT tax is greater than the regular tax, the taxpayer owes the total of the AMT tax and the regular tax.

The AMT calculation is parallel to a corporation's regular tax calculation.  This means a corporation must calculation tax under both system to determine  its TMT or Tentative Minimum Tax.

The TMT and RTL are then compared and if the TMT is higher than its regular  tax amount, the excess is the Corporation’s Alternative Minimum Tax (AMT).