The highest income tax rate which applies to your income.
For instance, if your income is £60,000, the amount between the personal allowance (currently £12,500) and £50,000 is taxed at the basic tax rate of 20% while anything between £50,001 and £60,000 is taxed at the higher tax rate of 40%. Your marginal tax rate is therefore 40%, and any income you were to earn above £60,000 would be taxed at this rate (until you reach £150,001 where the tax rate jumps to 45%).
