Marginal tax rate
The tax rate that applies to your next dollar of income, which is the rate on the topmost slice of what you earn rather than one applied to your income as a whole.
A progressive income tax, such as the federal income tax in the United States, is charged in slices. Each bracket taxes only the income falling inside it, and your marginal rate is the rate on the topmost slice. It is the price of the next dollar earned and the value of the next dollar deducted, which is why it is the rate almost every decision turns on.
That makes it the right rate for questions about change: what a deduction is really worth, whether a pre-tax contribution pays, what an exempt yield is worth against a taxable one. It is the wrong rate for describing your overall burden, which is the job of the effective tax rate.
The mistake is believing a raise can leave you worse off by pushing you into a higher bracket. Brackets alone cannot do that. Only the income above the threshold is taxed at the higher rate, and everything below it carries on being taxed exactly as before. What can genuinely raise the cost of an extra dollar, and now and then leave a household worse off for earning more, is a credit or benefit that shrinks or stops as income rises, which is a separate mechanism from the brackets. That is also why a stated bracket rate and a true marginal rate are not always the same number.