Tax Deduction
Also called: deduction, tax write-off, allowable expense
A tax deduction reduces the income you're taxed on, so its value equals the deduction times your marginal rate — $1,000 saves $220 at a 22% rate, not $1,000. This is the difference people most often get wrong when told an expense is 'a write-off'. Deductions are worth more to higher earners for exactly this reason, which is why credits are the preferred tool when policy aims to help lower incomes.
Formula
Tax saved = Deduction × Marginal tax rate