1, 3. Subtract, deduct express diminution in sum or quantity. To subtract suggests taking a part from a whole or a smaller from a larger: to subtract the tax from one's salary. To deduct is to take away an amount or quantity from an aggregate or total so as to lessen or lower it: to deduct a discount. Subtract is both transitive and intransitive, and has general or figurative uses; deduct is always transitive and usually concrete and practical in application.