Also called: premium year, lookback
Medicare premiums for a given year are based on the tax return from two years earlier. Income in the year you turn 63 sets your premiums for the year you turn 65.
SSA uses the most recent return the IRS can provide, normally from two years before the premium year. The consequence for planning is that a Roth conversion, home sale or other income spike raises premiums two years later, for one year.
In RetIQ you can enter your actual MAGI for the two most recent years so the first Medicare years of the projection use real figures.