Modified Adjusted Gross Income
AGI plus certain items added back. There is no single MAGI: each program defines its own. Medicare IRMAA and ACA subsidies both add back tax-exempt interest, so muni bond income still counts.
For Medicare IRMAA, MAGI is AGI plus tax-exempt interest. For ACA premium tax credits, it is AGI plus tax-exempt interest, plus the non-taxable part of Social Security, plus excluded foreign earned income.
Because Roth conversions and pre-tax withdrawals raise AGI, they raise every version of MAGI. Roth withdrawals raise none of them.