The technical term used on IRS Form 6251 ("Alternative Minimum Tax") for the result of the calculation to determine tax liability under the alternative minimum tax (AMT) system. If the tentative minimum tax is higher than the regular tax, the difference is added to the regular tax in a line for it on your regular tax return (e.g. IRS Form 1040); the sum is the amount of tax due. This sum of the regular tax plus any difference is the same as the AMT figure entered as tentative minimum tax. Therefore, to avoid confusion in an already too confusing area, this website uses the terms "alternative minimum tax" and "AMT" in all contexts, even if at times the discussion actually involves the tentative minimum tax.
See the sections ISOs: AMT and ISOs: AMT Advanced.