The Alternative Minimum Tax (AMT) attempts to ensure that individuals and corporations that benefit from certain exclusions, deductions or credits pay at least a minimum amount of tax. If you are not liable for AMT this year, but you paid AMT in one or more previous years, you may be eligible to take a minimum tax credit against your regular tax this year.
The minimum tax credit is allowed only for the AMT caused by deferral items, such as depreciation. You may also increase your minimum tax credit to the extent you had a qualified electric vehicle credit not allowed for 2013 solely because of tentative minimum tax limitations.
Publication 17, Your Federal Income Tax - Chapter 38, Other Credits - Under Nonrefundable Credits, see Nonrefundable Credit for Prior Year Minimum Tax