definition of deduct
deduct. 1  to take away, as from a sum or amount: Once you deduct your expenses, there is nothing left. 2  detract; abate (usually followed by from): The rocky soil deducts from the value of his property.
To deduct is simply to subtract. A tax deduction is a subtraction from your taxable income allowed by the government for certain expenses, which will result in your paying lower taxes. Your insurance deductible is the amount of a medical bill that the insurance company makes you subtract before it starts to pay--in other words, the amount that will come out of your own pocket. But deduction also means reasoning, and