1419, from L. deductus, pp. of deducere "lead down, bring away;" see deduce, with which it formerly was interchangeable. Technically, deduct refers to taking away portions or amounts; subtract to taking away numbers.
Main Entry: de·duct Function: transitive verb : to take away (an amount) from a total; specifically: to take as a deduction deducted —D. Q. Posin> —compare AMORTIZE