Marginal Tax Rate
The tax rate applied to your next dollar of income — the rate of your highest tax bracket.
Your marginal tax rate is the rate you'd pay on one more dollar of income. Because the U.S. uses graduated brackets, income is taxed in layers: the first chunk at the lowest rate, the next chunk higher, and so on. Your marginal rate is the rate of the top bracket your income reaches.
It's the rate that matters for decisions at the margin — whether to do a Roth conversion, realize a gain, or make a pre-tax contribution — because those actions add or subtract from your top layer of income. It is not the rate you pay on your whole income; that's your effective tax rate, which is lower.
This definition is general information to help you understand a term, not financial, tax, or legal advice. Figures that change year to year (limits, thresholds, rates) should be confirmed against current official sources. For guidance on your situation, a licensed fee-only fiduciary is the right next step.