Initialism standing for alternative minimum taxable income, which is similar in concept to adjusted gross income (AGI) for ordinary tax. When you exercise and hold ISOs and do not sell the stock in the year of exercise, the spread at exercise is part of AMTI. You subtract from AMTI your AMT exemption amount and calculate the amount of AMT. If it is higher than your ordinary tax, you pay the AMT amount (technically, AMT is the amount over your regular tax).
For more on these topics, see the sections ISOs: AMT and ISOs: AMT Advanced elsewhere on this website.